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"And thkrk ark uivntsiTiu op oPiRATioNt, but it is thk baiic God 















CflAPTBR I.*— Objects of Nutritiov 1 

Cbapter II. — Nutrition iv Vegetables 15 

§ 1. Food of Plants 15 

2. Absorption of Nutriment by Plants ....•«.. 19 

3. Exhalation 27 

4. Aeration of the Sap ^ 29 

5. Return of the Sap 36 

6. Secretion in Vegetables ^ . . 45 

7. Excretion in Vegetables .51 

Chapter HI. — ^Animal Nutrition in general 57 

S 1. Food of Animals 57 

2. Series of Vital Functions 69 

Chapter IV. — Nutrition in the lower orders of 
AxiiiALS 74 

Chapter V.-->NuTRiTioN in the higher orders op 
Aniiials 104 

Chapter VI.— Preparation of Food 113 

\ 1. Prehension of Liquid Food. ••••^....r.*.... 113 

2. Prehension of Solid Food 117 

3. Mastication by means of Teeth 140 


4. Fonnatkm and Developement of tbe Teeth • . 155 

5. Trituration of Food in Internal Cavities 167 

6. Deglutition 174 

7. Receptacles for retaining Food 178 

Chaptek VIL— >Diox8tiok 180 

Chapter VIII. — Chtlificatiok 203 

Cb AFTER IX.— Lacteal Absorption ,; 226 

Chapter X. — Circulation 229 

§ 1: Diffused Circulation 229 

2. Vascular Circulation 235 

3. Respiratory Circulation 265 

4. Distribution of Blood Vessels 281 

Chaptee: XL — Respiration 290 

% r. Respiration in general » 290 

2. Aquatk; respiration 293 

3. Atmospheric Respiration 310 

4. CbeMicat Changes etfected by Respiration • . 333 

Chaffer XII. — Secretion 342 

Chapter XIIL — Absorption « . 351 

Chapter XIV. — Nervous Power 354 


Chapter L — Sensation 362 

Chapter IL— Touch 377 

Chapter III. — Taste 393 



CH.*pTEa IV. — Smell ^ 396 

Chapter V. — Hearing .,...., 414 

§ 1. Acoustic Principles .•.. 414 

2. Physiology of Hearing in Man 420 

3. Comparative Physiology of Hearing • • • 434 

Chapter VI. — Vision 444 

§ I. Object of the Sense of Vision 444 

2. Modes of aocompUshing the objects of Vision 449 

3. Structure of the Eye ; 460 

4. Physiology of Perfect Vision 469 

5. Comparative Physiology of Vision • . • . 477 

Chapter VII. — Perception 508 

Chapter VIII. — Comparative Physiology of the 

Nervous System. • • 537 

§ 1. Nervous System of Invertebrated Animals. • . • 537 

2. Nervous System of Vertebrated Animals . . . . 553 

3. Functions of tlie Brain 561 

4. Compaiative Physiology of Perception 566 


Chapter L— Reproduction 581 

Chapter II. — Organic Developement 599 

Chapter III. — Decline op the System 619 

Chapter IV. — Unity op Design ....•• 625 

IsDEx 643 





Chapter I. 


The mechanical structure and properties of the 
oiganized fabric, which have occupied our atten-* 
fjon in the preceding volume, are necessary for 
the maintenance of life, and the exercise of the 
Tital powers. But however artificially that fabric 
may have been constructed, and however admi^ 
rable the skill and the foresight that have been 
displayed in ensuring the safety of its elaborate 
mechanism, and in preserving the harmony of 
hs complicated movements, it yet df necessity 
contains within itself the elements of its own dis* 
sdntion. The animal machine, in common with 



every other mechanical contrivance, is sabject 
to wear and deteriorate by constant nse. Not 
only in the greater movements of the limbs, but 
also in the more delicate actions of the internal 
oigans, we may trace the operation of many 
causes inevitably leading to their ultimate des- 
truction. Continued friction must necessarily 
occasion a loss of substance in the harder parts 
of the frame, and evaporation is constantly tending 
to exhaust the fluids. The repeated actions of 
the muscles induce certain changes in these 
organs, both in their mechanical properties and 
chemical composition, which impair their powers 
of contraction, and which, if suffered to continue, 
would, in no long time, render them incapable 
of exercising their proper Ainctions; and the 
same observation applies also to the nerves, and 
to all the other systems of oi^ans. Provision 
must accordingly be made for remedying these 
constant causes of decay by the supply of those 
peculiar materials, which the oi^ans require for 
recruiting their declining energies. 

It is obvious that the developement of the 
organs, and general growth of the body, must 
imply the continual addition of new particles 
from foreign sources. Organic increase consists 
tiot in the mere expansion of a texture previously 
condensed, and the filling up of its interstices 
by inorganic matter ; but tliei new materials that 
are added must, for this purpose, be incorporated 
with those which previously existed, and become 


identified with the liying substance. Thus we 
oAen find stnictores forming in the bodies of 
ammab of a nature totally different from that of 
tbe part from which they arise. 
In addition to these demands, a store of mate- 
riab is also wanted for the reparation of occa" 
siooal injmies, to which, in the course of its loiig 
weer^ the body is onaToidably exposed. Like 
I dup fitted out for a long voyage, and fortified 
against the various dangers of tempests, of ice- 
bergs, and of shoals, the animal system, when 
inmched into existence, should be provided with 
a store of such materials as may be wanted for 
tlie repair of accidental losses, and should also 
contain within itself the latent source of those 
eoogies, which may be called into action when 
demanded by the exigencies of the occasion. 

Any one of the circumstances above enume- 
nited would of itself be sufficient to establish the 
^neaeity of supplies of nourishment for tbe 
■ainteaance of life. But there are other consi^ 
iterations, equally important in a physiologiciBd 
pomt of view, and derived fixim the essential 
More of organization, which also produce a 
continual demand for these supplies ; and these 
I shall now endeavour briefly to explain. 

Constant and progressive change appears to 
be one of the leading characteristics of life ; . and 
^ materials ydach are to be endowed with vi« 
tditjr must therefore be selected and arranged 
^th a view to their continual modification, cor- 


responding to these ever Varying changes of con*' 
dition. The artificer, whose aim is to construct 
a machine for permanent use, and to secure it as 
much as possible from the deterioration arising 
from friction or other cases of injury,^ would, of 
course, make choice for that purpose of the most 
hard and durable materials, such as the metals, 
or the denser stones. In constructing a watch, for 
instance, he would form the wheels of brass; the 
spring and the barrel-chain of steel ; and for the 
pivot, where the motion is to be incessant, he 
would employ the hardest of all materials, — the 
diamond. Such a machine, once finished, being 
exempt from almost every natural cause of decay, 
might remain for an indefinite period in the 
same state. Far different are the objectii which 
must be had in view in the formation of organized 
structures. In order that these may be qualified 
for exercising the functions of life, they must be 
capable of continual alterations, displacements, 
and adjustments, vaiying perpetually, both in 
kind and in degree, according to the progressive 
stages of their internal developement, and to the 
different circumstances which may arise in their 
external condition. The materials which nature 
has employed in their construction, are, there^ 
fore, neither the elementary bodies, nor evea 
their simpler and more permanent combinations ; 
but such of their compounds as are of a more 
jplastic nature, and which allow of a variable 




propoitioQ of ingrediente, and of gveat divereity 
ID the modes of their combiufttion . So great is th^ 
eomplexity of these arrangements, that althou^ 
eheoiistry is fully competent to the analysis of 
flSanized substances into their ultimate elements^ 
no human art is adequate to offset their reuni<« 
in the same state as that in which they had 
existed in those substances ; for it was by the 
iefined operations of vitality, the only power that 
could produce this adjustment, that they have 
beeo brought into that condition. 

We may take as an example one of the simplest 
tf organic products, namely Sugar; a substance 
which has been analysed with the greatest accu- 
ncy by modem chemists : yet to reproduce this 
<ugar, by the artificial combination of its simple 
^ents, is a problem that has hitherto baffled 
dlthe efforts of philosophy. Chemistry, not- 
withstanding the proud rank it justly holds among 
^ physical sciences, and the noble discoveries 
*^ which it has enriched the arts ; notwith- 
^dmg it has unveiled to us many of the secret 
^^petations of nature, and placed in our hands 
*ooie of h^r most powerful instruments for acting 
ttpoQ matter ; and notwithstanding it is armed 
with full powers to destroy, cannot, in any one 
^^^Suuc product, rejoin that which has been once 
^^nerered. Through the medium of chemistry 
^c are enabled, perhaps, to form some estimate 
^ the value of what we find ex^ciit^ by other 


agencies ; bat the imitation of the model, even 
in the smallest part, is far beyond our power. 
No means which the laboratory can supply, no 
process, which the most inventive chemist can 
devise, have ever yet approached those delicate 
and refined operations which nature silently con* 
ducts in the organized texture of living plants 
and animals. 

The elements of organic substances are not 
very numerous; the principal of them being 
oxygen, carbon, hydrogen, nitrogen, sulphur, 
and phosphorus, together with a few of the alka- 
line, earthy, and metallic bases. These 8ub<^ 
stances are variously united, so as to form cer- 
tain specific compounds, which, although they 
are susceptible, in different instances, of endless 
modifications, yet possess such a general cha^ 
racter of uniformity, as to allow of their being 
arranged in certain classes ; the most character- 
istic substance in each class constituting what is 
called a proximate organic principle. Thus in 
the vegetable kingdom we have Lignin, TanniUy 
Mucilage 9 Oil^ Sugar ^ Fecula^ &c. The animal 
kingdom, in like manner, furnishes Gelatin^ 
Albumen^ Fibrin^ Mucus^ EntomolinCy Elearin^ 
Stearin^ and many others. 

The chemical constitution of these organic 
products, formed, aa they are, of but few pri-- 
mary elements, is strikingly contrasted with 
that of the bodies belonging to the mineral 


kiDgdmi. The catalogue of elementary^ or 
ample bodies, existing in nature, is, indeed^ 
more extensiye than the list of those which 
enter mto the composition of animal or vege- 
table substances. But in the mineral world 
diey occur in simpler combinations, resolvable, 
fcr the most part, into a few d^nite ingredients, 
which rarely comprise more than two or three 
dements. In organized products, on the other 
imd, although the total number of existing 
eiements may be smaller, yet the mode of com«- 
bbation in each separate compound is infinitely 
XNTe complex, and presents incalculable diver- 
9ty. Simple binary compounds are rarely ever 
met with ; but, in place of these, we find three, 
fear, five, or even a greater number of consti* 
taent elements existing in very complicated 
sbteg of union. 

This peculiar mode of combination gives rise 
^ a remarkable condition, which attaches to 
^ chemical properties of organic compounds. 

^e attractive forces, by which their several 


'I'gi^ients are held together, being very nume- 
^^ require to be much more nicely balanced, 
^cvder to retain them in combination. Slight 
^nses are sufficient to disturb, or even overset, 
^ equipoise of affinities, and often produce 
^d changes of form, or even complete decom- 
Potion. The principles, thus retained in a 
^^ of forced union, have a constant tendency 


to react upon one another, and to produce, from 
Alight variations of circumstances, a totally new 
iorder of combinations. Thus a degree of heat, 
which would occasion no change in most mineral 
.substances, will at once effect the complete di»- 
.union of the elements of an animal or vegetable 
body. Organic substances are, in like manner, 
unable to resist the slower, but equally destruc* 
tive agency of water and atmospheric air ; and 
they are also liable to various spontapeous 
changes, such as those constituting fermentation 
and putrefaction, which occur when their vitality 
is extinct, and when they are consequently 
abandoned to the uncontrolled operation of their 
jaatural chemical affinities. This tendency to 
decomposition may, indeed, be r^arded as 
inherent in all organized substances, and as 
requiring for its counteraction, in the living 
system, that perpetual renovation of materials 
which is supplied by the powers of nutrition. 

It would appear that during the continuance 
of life, the progress of decay is arrested at its 
very commencement; and that the particles, 
which first undergo changes unfitting them for the 
.exercise, of their functions, and which, if suffiared 
to remain, would accelerate the destruction of 
the adjoining parts, are immediately removed, 
and their place supplied by particles that have 
been modified for that purpose, and which, 
when they afterwards lose these salutary pro- 


f^eities, are in their turn discarded and replaced 
^y others. Hence the continued interchange 
and renewal of particles which take place in 
4he more active organs of the system, especially 
in the h^her classes of animals. In the fabric 
of those animals which possess an ext^isive 
system of circulating and absorbing vessels, the 
changes that are effected are so considerable and 
80 rapid, that even in the densest textures, such 
as the bones, scarcely any portion of the sub- 
stance which originally composed them i^ per* 
manently retained in their structure. To so 
great an extent is this renovation of materials 
earried on in the human system, that doubts 
may very reasonably be entertained as to the 
identity of any portion of the body after the 
Inpee of a certain time. The period assigned by 
the ancients for this entire change of the sub- 
stance of the body was seven or eight years : but 
modern inquiries, which show us the rapid re- 
paration that takes place in injured parts, and 
the quick renewal of the bones themselves, tend 
to prove that even. a shorter time than this is 
adequate to the complete renovation of every 
portion of the living fabric* 

Imperfect as is our knowledge of organic 
chemistry, we see enough to convince us that a 

• See the article ''Age-' in the Cjclopeedia of Practical 
Medicine, where I have enlarged ufon this subject. 


series of the most refined and artificial opera- 
tions is required in order to bring about the com^ 
plicated and elaborate arrangements of elements 
which constitute both animal and vegetable 
products. Thus in the very outset of this, as of 
every other inquiry in Phyriology, we meet with 
evidences of profound intention and consummate 
art, infinitely surpassing not only the power and 
resources, but even the imagination of man. 

Much as the elaborate and harmonious me- 
chanism of an animal body is fitted to excite our 
admiration, there can be no doubt that a more 
extended knowledge of that series of subtle pro* 
cesses, consisting of chemical combinations and 
decompositions which are continually going on 
in the organic laboratory of living beings, would 
reveal still greater wonders, and would fill us 
with a more fervent admiration of the infinite 


art and prescience which are even now mani- 
fested to us in every department both of the 
vegetable and animal economy. 

The processes by which all these important 
purposes are fiilfiUed comprise a distinct class of 
functions, the final object of which may be 
termed Nutrition^ that is, the reparation of the 
waste of the substance of the organs, their 
maintenance in the state fitting them for the 
exercise of their respective offices, and the appli- 
cation of properly prepared materials to their 
developement and growth. 


The fimctions subBervient to jratrition may be 
Astingnished, according bb the processeB they 
oomprise relate to seven principal periods in the 
natural order of their succession. The first 
series of processes has for its objects the re* 
eeption of the materials from without, and thdlr 
jM^qparation and gradual conversion into proper 
ootiiment, that is, into matter having the same 
chemical properties with the substance of the 
organs with which it is to be incorporated ; and 
their purpose being to assimilate the food bb 
much as possible to the nature of the organic 
body it is to nourish, all these functions have 
been included under the term Assimilation. 

The second series of vital functions com- 
prise those which are designed to convey the 
nutritive fluids thus elaborated, to all the organs 
that are to be nourished by them. In the more 
developed systems of organization this purpose 
m accomplished by means of canals, called vessels, 
tkroiigh which the nutritive fluids move in a 
kind of circuit : in this case the function is dcr 
nominated the Circulation. 

It is not enough that the nutritive juices are 
assimilated; another chemical process is still 
required to perfect their animalization, and to 
retain them in their proper chemical condition 
for the purposes of the system. This third object 
is accomplished by the function of Respiration. 

Fourthly, several chemical products which are 


wanted in different parts of the economy, are 
required to be formed by a peculiar set of organs, 
of which the intimate structure eludes observa- 
tion ; although we may perceive that in many 
instances, among the higher orders of beings, a 
special apparatus of vessels, sometimes spread 
over the surface of a membrane, at other times 
collected into distinct masses, is provided for 
that purpose. These specific organs are termed 
glandsj and the office performed by them, as 
well as by the simpler forms of structure above 
mentioned, is termed Secretion. 

Fifthly, similar processes of secretion are also 
employed to carry off from the blood such animal 
products as may have been formed or introduced 
into it, and may possess or have acquired noxious 
properties. The elimination of these materials, 
which is the office of the excretoriesy constitutes 
the function of Excretion. 

Sixthly, changes may take place in various 
parts of the body, both solid and fluid, rendering 
them unfit to remain in their present situation, 
and measures must be taken for the removal oi 
these useless or noxious materials, by transferring 
them to the general mass of circulating blood, 
so as either to be again usefiilly employed, 
or altogether discarded by excretion from die 
system. This object is accomplished by a 
peculiar set of vessels ; and the function they 
perform is termed Absorption. 


Lastly, the cony^rsion of the fluid nutrikdent 
into the solids of the body, and its immediate 
apfrfication to the purposes of the developement 
(dike organs, of their preservation in the state of 
health and activity, and of the repair of such 
iquries as they may chance to sustain, as far as 
lie powers of the systaooi are adequate to such 
nqiaration, are the objects of a seventh set of 
fQDctions, more especially comprised under the 
tide of Nutrition^ which closes this long series of 
cbemical changes, and this intricate but har* 
monious system of operations. 

Although the order in which the constituent 
d^oamts of organized products are arranged^ and 
the mode in which they are combined, are 
entirely imknown to us, we can nevertheless 
pecceiTe that in following them successively 
feun the simplest vegetables to the higher orders 
of the animal kingdom, they acquire continually 
increasing degrees of complexity, corresponding, 
in some measure^ to the greater refinement and 
complication of the structures by which they have 
been elaborated, and of the bodies to which they 
are ultimately assimilated. Thus plants derive 
their nourishment from the crude and simple 
materials which they absorb from the earth, the 
waters, and the air that surround them ; mate- 
rials which consist almost wholly of water, with 
a small proportion of carbonic acid, and a few 
saline ingredients, of which that water is the 


vehicle. But these, after having been converted 
by the powers of vegetable assimilation, into the 
substance of the plant, acquire the charac* 
teristic properties of organized products, though 
they are still the simplest of that class. In this 
state, and when the fabric they had composed is 
destroyed, and they are scattered over the soil, 
they are fitted to become more highly nutritive 
to other plants, which absorb them, and with 
more facility adapt them to the purposes of their 
own systems. Here they receive a still higher 
degree of elaboration ; and thus the same mate- 
rials may pass through several successive series 
of modifications, till they become the food of ani- 
mals, and are then made to undergo still fiirther 
changes. New elements, and in particular 
nitrogen, is added to the oxygen, hydrogen and 
carbon, which are the chief constituents of 
vegetable substances : * and new properties are 
acquired, from the varied combinations into 
which their elements are made to enter by the 
more energetic powers of assimilation apper- 
taining to the animal system. The products 
which result are still more removed from their 
original state of inorganic matter : and in this 
condition they serve as the appropriate food rf 

* Nitrogen, however, frequently enters into the composition 
of vegetables : though in general, in a much smaller proportion 
than into the substance of animals, of which Iset it always ap- 
pears to be an essential constkueat. 


camivorons animals, which generally hold a 
higber rank in the scale of organization, than 
tluse that subsist only on vegetables. 

Hius has each created being been formed in 
nference, not merely to its own welfare, but 
ab to that of multitudes of others which are 
(iq»endent oa it for their support, their preser- 
nOioD, — ^nay, even for their existence. In con- 
templating this mutual relationship, this sue- 
eenye subordination of the different races to one 
another, and this continual tendency to increased 
refinement, we cannot shut our eyes to the mag- 
aficent unfolding of the great scheme of nature 
ftr the progressive attainment of higher objects ; 
until, in the perfect system, and exalted endow- 
Beats of man, we behold the last result that has 
been manifested to us of creative power. 

Chapter II. 


§ 1 . Food of Plants. 

Tbe simplest kind of nutrition is that presented 
lo Qg by the vegetable kingdom, where water 
My be considered as the general vehicle of the 
Detriment received. Before the discoveries of 


modem chemiBtry it was very generally believed 
that plants could subsist on water alone; and 
Boyle and Van Helmont in particular endea- 
voured to establish by experiment the truth of 
this opinion. The latter of these physiologists 
planted a willow in a certain quantity of earth, 
the weight of which he had previously ascer- 
tained with great care ; and during five years, he 
kept it moistened with rain water alone, which 
he imagined was perfectly pure. At the end of 
this period he found that the earth had scarcely 
diminished in weight, while the willow had 
grown into a tree, and had acquired an ad- 
ditional weight of one hundred and fifty pounds : 
whence he concluded that the water had been 
the only source of its nourishment. But it does 
not seem to have been at that time known that 
rain water always contains atmospheric air, and 
frequently also other substances, and that it 
cannot, therefore, be regarded as absolutely pure 
water : nor does it appear that any precautions 
were taken to ascertain that the water actually 
employed was wholly free from foreign matter, 
which it is easy to conceive it might have held 
in solution. In an experiment of Duhamel, on 
the other hand, a horse-chesnut tree and an oak, 
exposed to the open air, and watered' with 
distilled water alone,- the former for three, and 
the latter for eight years, were kept alive, indeed; 
but they were exceedingly stinted in their growth^ 


Mid evidently derived little or no sustenance 
fiom the water with which they were supplied. 
Experiments of a similar nature were made by 
Bonnet, and with the like result. When plants 
nxe contained in closed vessels, and r^ularly 
sopplied with water, but denied all access to 
carbonic acid gas, they are developed only to a 
very limited extent, determined by tlte store of 
nutritious matter which had been already col- 
lected in each plant when the experiment com- 
menced, and which, by combining with the 
water, may have afforded a temporary supply 

But the water which nature famishes to the 
Tegetable organs is never perfectly pure ; for, be- 
sides containing air, in which there is constantly 
a certain proportion of carbonic acid gas, it has 
always acquired by percolation through the soil 
Tarious earthy and saline particles, togeth» with 
neiterials derived from decayed vegetable or 
animal remains. Most of these substances are 
flohible, in however minute a quantity, in water : 
and others, finely pulverized, may be suspended 
in that fluid, and carried along with it into the 
v^etable system. It does not appear, however, 
that pure carbon is ever admitted, for Sir H. Davy, 
on mixing charcoal, ground to an impalpable 
powder, with the water into which the roots of 
mint were immersed, could not discover that 
the smallest quantity of that substance had 

VOL. II. c 


been, in any caae» absorbed.* But in the form 
of carbonic acid, this element is received in 
great abundance, through the medium of water, 
which readily absorbs it : and a considerable 
quantity of carbon is also introduced into the 
fluids of the plant, derived from the decom- 
posed animal and vegetable materials, which the 
water generally contains. The peculiar fertility, 
of each kind of soil depends principally on the 
quantity of these organic products it contains in 
a state capable of being absorbed by the plant, 
and of contributing to its nourishment. 

The soil is also the source whence plants derive 
their saline, earthy, apd metallic ingredients. 
The silica they often contain is, in like manner, 
conveyed to them by the ws^ter, which it is now 
well ascertained, by the researches of Berzilius, 
is capable of dissolving a very minute quantify 
of this dense aud hard substance. It is evident 
that, however small this quantity may be, if it 
continue to accumulate in the plant, it may in 
time constitute the whole amount of that which 
is found to be so copiously deposited on the sur- 
face, or collected in the interior of many plants, 
such as the bamboo, and various species of grasses. 
The small d^ree of solubility of many substances 
thus required for the construction of (he solid 
vegetable fabric, is» probably, one of the reasons 
why plants require. so large a supply of water 
for their subsistence. 

* Elements of Agricukural Chemistry, Lect. VI ..p. 234* 


§ 2. Absorption of Nutriment by Plants. 


Tbm greater number of cellular plants absorb 
vater with nearly equal facility firbm every part 
rf their surface : this is the case with the Algmy 
ia instance,which are aquatic plants. InLickensi 
on the other hand, absorption takes plaee more 
partially ; but the particular parts of the surface 
where it occurs are not constantly the same, and 
appear to be determined more by mechanical 
causes than by any peculiarity of structure: 
some, however, are found to be provided in eertioB 
parts of the surface with stomata, which De 
CandoUe supposes may act as sudung orifices. 
Many mushrooms appear to be capable of al)^ 
aorbing fluids from all parts of their sur&ce 
indiscriminately ; and some species, again, are 
fimiished at their base with a kind of radical 
fibrils for that purpose. 

In plants haying a vascular structure, which 
is the case in by far the greater number, the 
roots are the special wgans to which this* office 
of absorbing nourishment is assigned : but it 
oocasionaUy happens that, under certain circum* 
stances, the leaves, or the stems of plants are 
found to absorb moisture, which they have been 
supposed to do by the stomata interspersed oh 
their surface. This, however, is not their batural 
action ; and they assume it only in forced situa- 


tions, when they procure no water by means of 
the roots, either from having been deprived of 
these organs, or fjpom their being left totally dry. 
Thus a branch, separated from the trunk, may 
be preserved from withering for a long time, if 
the leaves be immersed in water: and when the 
soil has been parched by a long drought, the 
dro(^ing plants will be very quickly revived by 
a shower of rain, or by artificial watering, even 
before any moisture can be supposed to have 
penetrated to the roots. 

It is by the extremities of the roots alone, or 
rather by the spongioles which are there situ- 
ated, that absorption takes place : for the surfoce 
of the root, being covered in every other part by 
a layer of epidermis, is incapable of performing 
this office. It was long ago remarked by. Du- 
hamel, that trees exhau^ the soil only in 
those parts which surround the extremities of 
the roots : but the fact that absorption is effected 
only at those points has been placed beyond a 
doubt by the direct experiments of Sennebier, 
who, taking two carrots of equal size, immersed 
in water the whole root of the one, while only 
the extremity of the other was made to dip into 
the water, and found that equal quantities were 
absorbed in both cases ; while on immersing the 
whole surface of another carrot in the fluid, with 
the exception of the extremity of the root, which 
was raised so as to be above the surface, no ab- 


s(»rption whatever took place. Plants having a 
fusiform^ or spindle-shaped root, such as the 
carrot and the radish, are the best for these ex- 

In the natural progress of growth, the roots 
are constantly shooting forwards in the direction 
they have first taken, whether horizontally, or 
vertically, or at any other inclination. Thus 
they continually arrive at new portions* of soil, 
€S which the nutritive matter has not yet been 
exhausted; and as a constant relation is {>re- 
served between their lateral extension and the 
Imizontal spreading of the branches, the greater 
part of the rain which falls upon the tree, is 
made to drop from the leaves at the exact dis- 
tance irom the trunk, where, after it has soiaked 
through the earth, it will be received by the ex- 
tremities of the roots, and readily sucked in by 
ttie spongioles. We have here a striking instance 
of that beautiful correspondence, which has been 
established between processes belonging to diffe- 
rent departments of nature, and which are made 


to concur in the production of remote effects, 
that could never have been accomplished without 
these preconcerted and harmonious adjustments. 
The spongioles, or absorbing extremities of 
the roots, are constructed of ordinary cellular or 
^ongy tissue : and they imbibe the fluids, which 
are in contact with them, partly by capillary 
action, and partly, also, by what has been termed 


a hygroscopic power. But though these principles 
may sufficiently account for the simple entrance 
of the fluids, they are inadequate to explain its 
continued ascent through the substance of the 
root, or along the stem of the plant. The most 
probable explanation of this phenomenon is that 
the progressive movement of the fluid is produced 
by alternate contractions and dilatations of the 
cells themselves, which compose the texture of 
the plant ; these actions being themselves refer^ 
able to the vitality of the organs. 

The absorbent power of the spongioles is 
limited by the diameter of their pores, so that 
fluids which are of too viscid or glutinous a con- 
sistence to pass readily through them are liable 
to obstruct or entirely block up these passages. 
Thus if the spongioles be surrounded by a thick 
solution of gum, or even of sugar, its pores will 
be clogged up, scarcely any portion of the fluid 
will be absorbed, and the plant will wither and 
perish : but if the isame liquids be more largely 
diluted, the watery portion will find its way 
through the spongioles, and become available 
for the sustenance of the plant, while the greater 
part of the thicker material will be left behind. 
The same apparent powe* of selection is exhibited 
when saline solutions of a certain strength are 
presented to the roots : the water of the solution, 
with only a small proportion of the salts, being 
taken up, and the remaining part of the fluid 


being found to be more Btrongly impregnated 
widi die salts than before this absorption had 
taken place. It would appear, however, that all 
this is merdy the result of a mechanical opera- 
tion, and that it furnishes no evidence of any 
fiscriminating faculty in the qK>ngiole : for it is 
found liiat, provided the material presented be 
in a state of perfect solution and limpidity, it is 
sacked in with equal avidity, whether its qualities 
be del^erious or salubrious. Solutions of sul- 
pJiale of copper, which is a deadly poison, are 
absorbed in laige quantities by the roots of plants, 
vhich are immersed in them : and water which 
drains from a bed of manure, and is consequently 
loaded with carbonaceous particles, proves ex* 
ceedinglyinjurious when admitted into the system 
of the plant, from the excess of nutriment it con- 
tains. But in the ordinary counde of v^etation, 
no danger can arise from this general power of 
abeorptiori, since the fluids which nature supplies 
are always such as are suitable to the organs 
that are to receive them. 

The fluid, which is taken up by the roots, and 
which, as we have seen, consists chiefly of water, 
holding in solution atmospheric air, together 
with various saline and earthy ingredients neces- 
sary for the nourishment of the plant, is in a 
p^ecdy crude state. It rises in the stem of 
the plant, undei^oing scarcely any perceptible 
change in its ascent ; and is in this state conducted 


to the leaves, where it is to experience variote 
important modifications. By causing the roots 
to imbibe coloured liquids, the general course of 
the sap has been traced with tolerable accuracy, 
and it is found to traverse principally the ligneous 
substance of the stem : in trees, its passage is 
chiefly through the alburnum, or more recently 
formed wood, and not through the bark, as was 
at one time believed. 

The course of the sap, however, varies under 
different circumstances, and at different epochs 
of vegetation. At the period when the young 
buds are preparing for their developement, which 
usually takes place when the genial warmth of 
spring has penetrated beyond the surface, and 
expanded the fibres and vessels of the plant, 
there arises an urgent demand for nourishment, 
which the roots are actively employed in sup- 
plying. As the leaves are not yet completed, the 
£ap is at first applied to purposes somewhat 
different fi-om those it is destined to fulfil at a 
more advanced period, when it has to nourish 
the fully expanded organs: this fluid has, ac- 
cordingly, received a distinct appellation, being 
termed the nursling sap. Instead of rising 
through the alburnum, the nursling sap ascends 
through the innermost circle of wood, or that 
which is immediately contiguous to the pith, and 
is thence transmitted, by iinknown channels^ 
through the several layers of wood, till it reaches 


the buds, which it is to supply with nourishment. 
During this circuitous passage, it probably un- 
dergoes a certain degree of elaboration, fitting it 
fiur the office which it has to perform : it appa«- 
rently combines.with some nutriment, which had 
been previously deposited in the plant, and which 
it again dissolves ; and thus becoming Msimilated^ 
is in a state proper to be incorporated with the 
new organization that is developing* This nurs- 
ling sap, provided for the nourishment of the 
young buds, has been compared to the milk of 
animals, which is prepared for a similar purpose 
at those times only when nutriment is required 
Ibr the rearing of their young. 

Several opinions have been entertained with 
regard to the channels through which the sap is 
conveyed in its ascent along the stem, and in 
its passage to its ultimate destination. Many 
observations tend to show, that, in ordinary cir- 
cumstances, it is not transmitted through any of 
the distinguishable vessels of the plant : for most 
of these, in their natural state, are found to con- 
tain only air. The sap must, therefore, either 
tsraverse the cells themselves, or pass along the 
intercellular spaces. That the latter is the 
course it takes is the opinion of De Candolle, 
who adduces a variety of arguments in its sup- 
pwt. The sap, he observes, is found to rise 
equally well in plants whose structure is wholly 
ceUukur ; a fact which proves the vessels are not 


in all cases necessary for its conveyance. In 
many instances the sap is known to deviate from 
its usual rectilinear path, and to pursue a cir«- 
cuitous course, very diiferent from that of any of 
the known vessels of the plant. The diffusion 
of the sap in different direction^, and its sub* 
sidence in the lowest parts, on certain occasions, 
are facts irreconcileable with the supposition 
that it is confined in these vessels. 

Numerous experiments have been made to 
discover the velocity with which the sap rises in 
plants, and the fOTce it exerts in its ascent 
Those of Hales are well known : by lopping off 
the top of a young vine, and applying to the 
truncated extremity a glass tube, which closed 
Totmd it, he found that the fluid in the tube rose 
to a height, which, taking into account the qpecific 
gravity of the fluid, was equivalent to a perpendi- 
cular column of water of more than forty-thr« 
feet ; and consequently exerted a force of propul- 
sion considerably greater than the pressure of an 
additional atmosphere. The velocity, as well as 
the force of ascent, must, however, be liable to 
great variation; being much influenced by eva- 
poration, and other changes, which the sap 
undergoes in the leaves. Various opinions have 
been entertained as to the agency by which the 
motion of the sap is effected ; but although it 
seems likely to be resolved into the vital move- 
nfrents of the cellular structure already mentioned, 


tile question is still enveloped in considerable 
obflCQiity. There is certainly no evidence to 
prove that it has any analc^ to a muscular 
power; and the simplest supposition we can 
make is that these actions take place by means 
(^a contractile prcq>erty belonging to the vege- 
table tissue, and exerted, under certain circum- 
stances, and in conformity to certain laws, which 
ve have not yet succeeded in determining. 

^ 3. ExhalatioM, 

The nutrient sap, which, as we have seen, rises 
in the stem, and is transmitted to the leaves 
witbout any change in its qualities or compo- 
sitkm, is immediately, by the medium of the 
fltomata, or orifices which abound in the surface 
of those organs, subjected to the process of 
cthdation. The proportion of water which the 
sap loses by exhalation in the leaves is generally 
about two-thirds of the whole quantity received-; 
to that it is only the remaining third that returns 
toooarishthe organs of the plant. It has been 
ascertained that the water thus evaporated is 
perfectly pure ; or at least does not contain more 
than a 10,000,000th part of the foreign matter 
with which it was impregnated when first ab- 
sorbed by the roots. The water thus exhaled. 


being dissolved by the air the moment it escapes, 
passes off in the form of invisible vapour. Hales 
made an experiment with a sun-flovirer, three 
feet high, enclosed in a vessel, which he kept for 
fifteen days : and inferred from it that the daily 
loss of the plant by exhalation was twenty ounces; 
and this he computes is a quantity seventeen 
times greater than that lost by insensible perspi- 
ration from an equal portion of the surface of the 
human body. 

The comparative quantities of fluid exhaled 
. by the same plant at different times are regu* 
lated, not so much by temperature, as by the 
intensity of the light to which the leaves are 
exposed. It is only during the day, therefore, 
that this function is in activity. De Candolie 
has found that the artificial light of lamps pro- 
duces on the leaves an effect similar to that of 
the solar rays, and in a d^ree proportionate to 
its intensity.* As it is only through the stomata 
that exhalation proceeds, the number of these 
pores in a given surface must considerably in- 
fluence the quantity of fluid exhaled. 

By the loss of so large a portion of the water 
which, in the rising sap, had held in solution 
various foreign materials, these substances are 
rendered more disposed to separate from the 
fluid, and to become consolidated on the sides 

• Physiologic Vegetale, i. 112. 


of the cells or vessels, to which they are con- 
docted from the leaves. This, then, is the first 
modification in the qualities of the sap which it 
undergoes in those organs. 

§ 4. Aeration of the Sap. 

A CHEMICAL change much more considerable 
and important than the preceding is next effected 
on the sap by the leaves, when they are subjected 
to the action of light. It consists in the decom- 
position of the carbonic acid gas, which is either 
brought to them by the sap itself, or obtained 
directly from the surrounding atmosphere. In 
either case its oxygen is separated, and is dis- 
engaged in the form of gas ; while its carbon is 
retained, and composes an essential ingredient 
of the altered sap, which, as it now possesses one 
of the principal elements of vegetable structures, 
may be considered as having made a near ap- 
proach to its complete assimilation j using this 
term in the physiological sense already pointed 

The remarkable discovery that oxygen gas is 
exhaled from the leaves of plants during the 
day time, was made by the great founder of 
pBemnatic chemistry, Dr. Priestley : to Senne- 
bier we are indebted for the first observation 


that the presence of carbonic acid is required 
for the disengagement of oxygen in this proceas, 
and that the oxygen is derived from the decom- 
position of the carbonic acid ; and these lattor 
facts have since been fully established by the 
researches of Mr. Woodhouse, of Pensylvania, 
M. Theodore de Saussure, and Mr. Palmer. 
They are proved in a very satisfactory manner 
by the following experiment of De Candolle. 

Two glass jars were inverted over the same 
water-bath ; the one filled with carbonic acid 
gas, the other filled with water, containing a 
sprig of mint ; the jars communicating below by 
means of the water-bath, on the surface of whicb. 
some oil was poured, so as to intercept all com- 
munication between the water and the atmosphere. 
The sprig of mint was exposed to the light of the 
sun for twelve days consecutively : at the end 
of each day the carbonic acid wps seen to dimi- 
nish in quantity, the water rising in the jar to 
supply the place of what was lost, and at the 
same time the plant exhaled a quantity of 
oxygen exactly equal to that of the carbonic 
acid which had disappeai*ed. A similar sprig of 
mint, placed in a jar of the same size, full of dis- 
tilled water, but without having access to carbonic 
acid, gave out no oxygen gas, and soon perished. 
When, in another experiment, conducted by 
means of the same apparatus as was used in the 
first, oxygen gas was substituted in the first jar 


instead of carbonic acid gas, no gas was disen- 
gaged in the other jar, which contained a sprig 
of mint. It is evident, therefore, that the oxygen 
gas obtained from the mint in the first experi- 
ment was derived from the decomposition, by 
the leaves of the mint, of the carbonic acid,which 
the plant had absorbed from the water. 

Solar light is an essential agent in effecting 
this chemical change ; for it is never found to 
take place at night, nor while the plant is kept 
in the dark. The experiments of Sennebier 
would tend to show that the violet, or most re- 
firangible of the solar rays have the greatest 
power in determining this decomposition of car- 
bonic acid : but the experiments are of so deli- 
cate a nature, that this result requires to be con- 
finned by a more rigid investigation, before it 
can be adndtted as satisfactorily established. 

That the carbon resulting from this decompo- 
sition of carbonic acid is retained by the plant, 
has been amply proved by the experiments of 
M. Theodore de Saussure, who found that this 
process is attended with a sensible increase in 
the quantity of carbon which the plant had pre- 
viously contained. 

It is in the green substance of the leaves alone 
that this process is conducted ; a process, which, 
from the strong analogy that it bears to a similar 
function in animals, may be considered as the 
respiration of vegetables. The effect appears to 


be proportionate to the number of stomata which 
the plant contains. It is a process which takes 
place only in a living plant ; for if a leaf be 
bruised so as to destroy its organization, and 
consequently its vitality, its substance is no longer 
capable either of decomposing carbonic acid gas 
under the influence of solar light, or of absorbing 
oxygen in the dark. Neither the roots, nor the 
flowers, nor any other parts of the plant, which 
have not this green substance at their surface, 
are capable of decomposing carbonic acid gas : 
they produce, indeed, an eflect which is in some 
respects the opposite of this ; for they have a 
tendency to absorb oxygen, and to convert it 
into carbonic acid, by uniting it with the carbon 
they themselves contain. This is also the case 
with the leaves themselves, whenever they are 
not under the influence of light : thus, during 
the whole of the night, the same leaves, which 
had been exhaling oxygen during the day, ab- 
sorb a portion of that element. The oxygen 
thus absorbed enters immediately into combina- 
tion with the carbonaceous* matter in the plant, 
forming with it carbonic acid ; this carbonic acid 
is in part exhaled ; but the greater portion either 
remains attached to the substance of the leaf, or 
combines with the fluids which constitute the 
sap : in the latter case it is ready to be again 
presented to the leaf, when daylight returns, 
and when a fresh decomposition is again effected. 


This reversal at night of what was done in the 
day may, at first sight, appear to be at variance 
with the unity of plan, which we should ex- 
pect to find preserved in the vegetable economy : 
but a more attentive examination of th6 process 
will show that the whole is in perfect harmony, 
and that these contrai'y processes ate both of 
them necessary, in order to produce the result 

The water which is absorbed by the roots 
generally carries with it a certain quantity of 
soluble animal or vegetable materials^ which con- 
tain carbon. This carbon is transmitted to the 
leaves, where, during the night, it is made to 
combine with the oxygen they have absorbed. 
It is thus converted into carbonic acid, which, 
when daylight prevails, is decomposed ; the 
oxygen being dissipated, and the carbon retained. 
It is evident that the object of the whole process 
is to obtain carbon in that precise state of disin- 
tegration, to which it is reduced at the moment 
of its separation from carbonic acid by the action 
of solar ^ight on the green substance of the 
leaves ; for it is in this state alone that it is avail- 
able in promoting the nourishment of the plant, 
and not in the crude condition in which it exists 
when it is pumped up from the earth, along with 
the water which conveys it into the interior of 
the plant. Hence the necessity of its having to 
undergo this double operation of first combining 



with oxygen, and then being precipitated from 
its combination in the manner above described. 
It is not the whole of the carbon introduced into 
the v^etable system, in the form of carbonic 
acid, which has to undergo the first of these 
changes, a part of that carbon being already in 
the condition to which that operation would re- 
duce it, and consequently in a state fit to receive 
the decomposing action of the leaves. The 
whole of these chemical changes may be included 
under the general term Aeration. 

Thus the great object to be answered by this 
vegetable aeration is exactly the converse of 
that which we shall afterwards see is effected by 
the respiration of animals : in the former it is 
that of adding carbon, in an assimilated state, to 
the vegetable organization ; in the latter, it is 
that of discharging the superfluous quantity of 
carbon from the animal system. The absorption 
of oxygen, and the partial disengagement of 
carbonic acid, which constitute the nocturnal 
changes effected by plants, must have a tendency 
to deteriorate the atmosphere with respect to its 
capability of supporting animal life ; but this . 
effect is much more than compensated by the 
greater quantity of oxygen given out by the same 
plants during the day. On the whole, therefore, 
the atmosphere is continually receiving fix>m the 
vegetable kingdom a large accession of oxygen, 
and is, at the same time, freed from an equal 
portion of carbonic acid gas, both of which eflGscts 


tend to its purification and to its remaining 
adapted to the respiration of animals. Nearly 
the whole of the carbon accumulated by vege- 
tables is so much taken fiom the atmosphere, 
which is the primary source from which they 
derive that element. At the season of the year 
when vegetation is most active, the days are 
kmger than the nights ; so that the diurnal pro- 
cess of purification goes on for a greater number 
of hours than the nocturnal process by which the 
air is vitiated. 

The oxygen given out by plants, and the car- 
bonic acid resulting from animal respiration, 
and from the various processes of combustion, 
which are going on in every part of the world, 
are quickly spread through the atmosphere, not 
only from the tendency of all gases to uniform 
diffusion^ but also from the action of the winds, 
which are continually agitating the whole mass, 
and promoting the thorough mingling of its dif- 
(erexkt portions, so as to render it perfectly ho- 
mogeneous in every region of the globe, and at 
every elevation above the surface. 

Thus are the two great organized kingdoms of 
the creation made to co-operate in the execution 
of the same design: each ministering to the 
oth^, and preserving that due balance in the 
constitution of the atmosphere, which adapts it 
to the welfare and activity of every order of 
bemgs, and which would soon be destroyed, were 
the operations of any one of them to be sus- 


pended. It is impossible to contemplate sO 
special an adjustment of opposite effects without 
admiring this beautiful dispensation of Provi- 
dence, extending over so vast a scale of beings 
and demonstrating the unity of plan on which 
the whole system of organized creation has been 

§ 5. Return oftlie Sap. 

The sap, which, during its ascent from the roots, 
contains but a small proportion of nutritious par-^' 
tides, diluted with a large quantity of water^ 
after undergoing in the leaves, as in a chemical 
laboratory, the double processes of exhalation 
and aeration, has become much more highly 
charged with nutriment ; and that nutriment has 
been reduced to those particular forms and states 
of composition which render it applicable to the 
growth of the organs, and the other purposes of 
vegetable life. This fluid, therefore, corresponds 
to tlie blood of animals, which, like the elaborated 
wip, may be regarded as fluid nutriment, per- 
fectly assimilated to that particular kind of or- 
ganization, with which it is to be afterwards in- 
corporated. From the circumstance of its being 
sent back from the leaves for distribution to the 
several organs where its presence is required, it 
has received the name of the returning mp, that 
it might be distinguished from the crude fluid 


which arrives at the leaves, and which is termed 
the ascending sap. 

The returning sap still contains a considerable 
quantity of water, in its simple liquid form; which 
was necessary in order that it might still be the 
vehicle of various nutritive materials that are 
dissolved in it. It appears, however, that a large 
proportion of the water, which was not ex- 
haled by the leaves, has been actually decom- 
posed, and that its separated elements, the oxygen 
and the hydrogen, have been combined with 
certain proportions of carbon, hydrogen, nitrogen, 
and various earths, metals, and salts, so as to 
finrm the proximate v^etable products, which 
are found in the returning sap. 

The simplest, and generally the most abundant 
of these products, is that which is called Gum.* 
From the uiiiversal presence of this substance 
in the vegetable juices, and ^ more especially 
in the returning sap, of all known plants, from 
its hland and unirritating qualities, from its great 
solubility in water, and from the facility with 
which other vegetable products are convertible 
into this product, Gum may be fairly assumed 

* According to the investigations of Dr. Prout, 1000 grains 
of gum are composed of 586 grains of the elements of water, that 
is, of oxygen and hydrogen, in the exact proportions in which 
they would have united to form 5^6 grains of water ; together 
with 414 of carbon, or the base of carbonic acid. This, accord- 
ing to the doctrine of chemical equivalents, corresponds to one 
molecule of water, and one molecule of carbon. Phil. Trans. 
1827, 584. 


to be the principal basis of vegetable nutriment ; 
and its simple and definite composition points 
it out as being the immediate result of the che- 
mical changes *which the sap experiences in the 
leaves. During the descent of the sap, however, 
' this fluid undergoes, in various parts of the plant, 
a further elaboration, which gives rise to other 
products. We are now, therefore, to follow it in 
its progress through the rest of the vegetable 

The returning sap descends firom the leaves 
through two different structures: in exogenous 
plants the greater portion finds a ready passage 
through the liber, or innermost layer of bark, 
and another portion descends through the albur- 
num, or outermost layer of the wood. With re- 
gard to the exact channels through which it 
passes, the same degree of uncertainty prevails 
as with regard to those which transmit the as- 
cending sap. De CandoUe maintains that, in 
either case, the fluids find their way through the 
intercellular spaces : other physiologists, how- 
ever, are of opinion, that particular vessels are 
appropriated to the ofiice of transmitting the des- 
cending sap. The extreme minuteness of the 
organs of vegetables has hitherto presented 
insuperable obstacles to the investigation of this 
important question : and consequently our rea- 
sonings respecting it can be founded only on 
indirect evidence. The processes of the animal 


economy, where the channels of distribution, 
and the organs of propulsion are plainly obser- 
vable» afford but imperfect analogies to guide us 
IB this intricate inquiry : for although it is true 
that in the higher classes of animals the circula- 
tiofi of the nutrient fluid, or blood, through dis- 
tinct vessels, is dufficiently obvious, yet in the 
lower departments of the animal kingdom, and 
in the embryo condition even of the more perfect 
species, the nutritious juices are distributed with- 
out being confined within any visible vessels; 
and they either permeate extensive cavities in 
the interior of the body, or penetrate through 
the interstices of a cellular tissue. That this latter 
is the mode of transmission adopted in the vege- 
table system baabeen considered probable, from the 
circomstance that the nutritious juices are diffused 
throughout those plants which contain no vessels 
whatsoever with the same facility as throughout 
those which possess vessels ; from which it has 
been concluded that vessels are not absolutely 
necessary for the performance of this ftmction. 
The nature of the forces which actuate the sap 
in its descent from the leaves, and its distribu- 
tion to different parts, is involved in equal ob- 
scurity with the nature of the powers which 
contribute to its motion upwards along the stem, 
firom the roots to the leaves. In endogenous 
plants the passage of the sap in its descent, is, in 
like manner, through those parts which have 


been latest formed ; that is, through the inner- 
most layers of their structure. 

The returning sap, while traversing these se- 
veral parts of the plant, deposits in each the par- 
ticular materials which are requisite for their 
growth, and for their maintenance in a healthy 
condition. That portion which flows along the 
liber, not meeting with any ascending stream of 
fluid, descends without impediment to the roots, 
to the extension of which, after it has nourished 
the inner layer of bark, it particularly contri- 
butes : that portion, on the other hand, which 
descends along the alburnum, meets with the 
stream of ascending sap, which, during the day 
at least, is rising with considerable force. A 
certain mixture of these fluids probably now 
takes place, and new modifications are in conse- 
quence produced, which, from the intricacies of 
the chemical processes thus conducted in the 
inner recesses of vegetable organization, we are 
utterly baffled in our attempts to follow. All 
that we are permitted to see are the general re- 
sults, namely the gradual deposition of the mate- 
rials of the future alburnum and liber. These 
materials are first deposited in the form o( a 
layer of glutinouB substance, termed the Cam- 
bium; a substance which appears to consist of 
the solid portion of the sap, precipitated from it 
by the separation of the greater part of the water 
that held it in solution. The cambium becomes 
in process of time more and more consolidated, 


and acquires the civilization proper to the plant 
of which it now forms an int^rant part : it con- 
stitutes two layers, the one, belonging to the 
wood, being the alburnum ; the other, bdonging 
to the bark, being the liber. 

The alburnum and the liber, which have been 
thus constructed, perform an important part in in- 
dacing ulterior changes on the nutrient materials 
which the returning sap continues to supply* 
Their cells absorb the gummy substance from 
the surrounding fluid, and by their vital powers 
effect a still further daboration in its compo* 
sition ; converting it either into starchy or sugar, 
or lignin, according to the mode in which its 
constituent elements are arranged. Although 
these several principles possess very different 
sensible properties, yet they are found to differ 
but very slightly in the proportions of their in- 
gredients ; &nd we may infer that the real che- 
mical alterations, which are required in order to 
effect these conversions, are comparatively slight, 
and may readily take place in the simple cellular 

In the series of decompositions which are arti- 

• According to the analyses of Dr. Prout, the following is the 
composition of these substances : 1000 parts of 
Pure Gum Arabic consist of 586 of oxygen and hydrogen, 

united in the proportions in which they exist in water, and 

414 of carbon. 
Dried Starch or Fecula of 560 water, and 440 carbon. 

Pure crystallized Sugar . . 572 428 

Lignio from Boxwood .. . 500 - - 500 - - - - 


ficially effected in the laboratory of the chemist, 
it has been found that gum and sugar are inter- 
mediate products, or states of transition between 
various others ; and they appear to be peculiarly 
calculated, from their great solubility, for being 
easily conveyed from one organ to another. S tarch, 
and lignin, on the other hand, are compounds of 
a more permanent character, and especially 
adapted for being retained in the organs. Starch 
which, though solid, still possesses considerable 
solubility, is peculiarly fitted for being applied 
to the purposes of nourishment : it is accord- 
ingly hoarded in magazines, with a view to 
future employment, being to vegetables, what 
the fat is to animals, a resource for the exigencies 
that may subsequently arise. With this inten- 
tion, it is carefully stored in small cells, the coats 
of which protect it from the immediate dissolving 
action of the surrounding watery sap, but allow of 
the penetration of this fluid, and of its solution, 
when the demands of the system require it. The 
tuberous root of the potatoe, that invaluable gift 
of Providence to the human race, is a remark- 
able example of a magazine of nutritive matter 
of this kind. 

The lignin, on the contrary, is deposited with 
the intention of forming a permanent part of the 
vegetable structure, constituting the basis of the 
woody fibre, and giving mechanical support and 
strengtli to the whole fabric of the plant. These 


latter structures may be compared to the bones 
of animals, composing by their union the solid 
firame work, or skeleton of the organized system. 
The woody fibres do not seem to be capable of 
&rther alteration in the living vegetable, and 
are never, imder any circumstances, taken up 
and removed to other parts of the system, as is 
the case with nutritive matter of a more conver- 
tible kind. 

The sap holds in solution, besides carbona- 
ceous matter, some saline compounds and a few 
earthy and metallic bases : bodies which, in how- 
ever minute a quantity they may be present, 
have unquestionably a powerful influence in 
determining certain chemical changes among 
the elements of organic products, and in im- 
parting to them peculiar properties ; for it is now 
a well ascertained fact that a scarcely sensible 
portion of any one ingredient is capable of pro- 
ducing important differences in the properties of 
the whole compound. An example occurs in 
the case of gold, the ductility of which is totally 
destroyed by the presence of a quantity of either 
antimony or lead, so minute as barely to amount 
to the two thousandth part of the mass ; and even 
the fumes of antimony, when in the neighbour- 
hood of melted gold, have the power of destroy- 
ing its ductility.* In the experiments made by 

♦ Hatchett. 


Sir John Herschel on some remarkable motions 
excited in fluid conductors by the transmission 
of electric currents, it was found that minute 
portions of calcareous matter, in some instances 
less than the millionth part of the whole com- 
pound, are sufficient to communicate sensible 
mechanical motions, and definite properties to 
the bodies with which they are mixed.* 

As Silica is among the densest and least soluble 
of the earths, we might naturally expect that 
any quantity of it taken into the vegetable 
system in a state of solution, would very early 
be precipitated from the sap/ after the exhala- 
tion of the water which held it dissolved ; and it 
is found, accordingly, that the greater portion of 
this silica is actually deposited in the leaves, 
and the parts adjacent to them. When once 
deposited, it seems incapable of being again 
taken up, and transferred to other parts, or 
ejected from the system : and hence, in course 
of time, a considerable accumulation of silicious 
particles takes place, and by clogging up their 
cells and vessels, tends more and more to ob- 
struct the passage of nourishment into these 
organs. This change has been assigned as a 
principal cause of the decay and ultimate des- 
truction of the leaves: their foot-stalks, more 
especially suffering from this obstruction, perish, 

♦ Philosophical Transactions for 1824, p. 162. 


and occasion the detachment of the leavesi 
which thus fall off at the end of each season, 
making way for those that are to succeed them 
io the next. 

§ 6. Secretion in Vegetables. 

While the powers of the simpler kinds of cells 
are adequate to produce in the returning sap the 
modifications ahove described, by which it is 
eonverted into gummy, saccharine, amylaceous, 
or ligneous products ; there are other cellular 
oigans, endowed with more extensive powers of 
chemical action, which effect still greater changes. 
The nature of the agents by which these changes 
are produced are unknown, and are therefore 
referred generally to the vital energies of vege* 
tation; but the process itself has been termed 
Secretion^ and the organs in which it is conducted, 
and which are frequently very distinguishable 
as separate and peculiar structures, are called 
Glands. When the products of secretion are 
chemically analysed, the greater number are 
foimd to contain a large quantity of hydrogen, 
in addition to that which is retained in combi- 
nation with oxygen as the representative of 
water: this is the case with all the oily secre- 
tions, whether they be fixed or volatile, and also 
with those secretions which are of a resinous 


quality. Some, on the contrary, are found to 
have an excess of oxygen ; and this is the condi- 
tion of most of the acid secretions ; while others, 
again, appear to have acquired an addition of 

All these substances have their respective uses, 
although it may frequently be difficult to assign 
them correctly. Some are intended to remain per- 
manently inclosed in the vesicles where they were 
produced ; others are retained for the purpose of 
being employed at some other time ; while those 
belonging to a third class are destined to be 
thrown off from the system as being superfluous 
or noxious : these latter substances, which are 
presently to be noticed, are specially designated 
as excretionsr Many of these fluids find their 
way from one part of the plant to another, with- 
out appearing to be conducted along any definite 
channels, and others are conveyed by vessels, 
which appear to be specially appropriated to this 

The following are examples of the uses to which 
the peculiar secretions of plants are applied. 
Many lichens, which fix themselves on calcareous 
rocks, such as the Patellaria immersa, are ob- 
served, in process of time, td sink deeper and 
deeper beneath tlie surface of the rock, as if 
they had some mode of penetrating into its sub- 
stance, analogous to that which many marine 
worms are known to possess. The agent appears 


in both instsdices to be an acid, which here is 
probably the oxalic, acting upon the carbonate 
of lime, and producing the gradual excavation 
of the rock. This view is confirmed by the ob* 
servation that the same species of lichen, when 
attached to rocks which are not calcareous, re- 
mains always at the surface, and does not pene- 
trate below it. 

A caustic liquor is sometimes collected in 
Tesicles, situated at the base of slender hairs, 
having a canal which conducts the fluid to the 
point. This is the case with the Nettle, The 
slightest pressure made by the hand on the hairs 
growing on the leaves of this plant, causes the 
fluid in their vesicles to pass out from their points, 
so as to be instilled into the skin, and occasion 
the well known irritation which ensues. M. De 
CandoUe junior has ascertained by chemical 
tests that the stinging fluid of the nettle is of an 
alkaline nature. In some species of this genus 
of plants, the hairs are so large that the whole 
mechanism above described is visible to the 
naked eye. This apparatus bears a striking re- 
aemblatice to that which exists in the poisonous 
teeth of serpents, and which is hereafter to be 

As the resinous secretions resist the action of 
water, we &ad them often employed by nature 
as a means of efiectually defending the young 
buds from the injurious efiects of moisture ; and 


for a similar purpose we find the surface of many 
plants covered with a varnish of wax, which is 
another secretion belonging to the same class : thus 
the Cei'oxyhnj and the Iriartea have a thick 
coating of wax, covering the whole of their stems. 
Sometimes the plant is strewed over with a bluish 
powder, possessing the same property of repelling 
water : the leaves of the Mesembryanthemum, or 
Fig-marigold, of the Atriplex^ or Orache, and of 
the Brassica^ or Cabbage, may be given as ex- 
amples of this curious provision. Such plants, 
if completely immersed in water, may be taken 
out without being wetted in the slightest d^ree ; 
thus presenting us with an analogy to the plu* 
mage of the cygnet, and other aquatic . birds, 
which are rendered completely water-proof by 
an oily secretion spread over their surface. 
Many aquatic plants, as the Batrachospe^mum^ 
are, in like manner, protected by a viscid layer, 
which renders the leaves slippery to the feel, and 
which is impermeable to water. 

Several tribes of plants contain liquids that 
are opaque, and of a white milky appearance ; 
this is the case with the Pappy y the Fig-tree^ the 
CanvolvuluSf and a multitude of other genera ; 
and a similar kind of juice, but of a yellow 
colour, is met with in the Chelidanium^ or Celan- 
dine. All these juices are of a resinous nature, 
and usually highly acrid, and even poisonous in 
their qualities; and their opacity is occasioned 


by the presence of a great number of minute 
globules, yisible with the microscope. The vessels 
in which these fluids are contained are of a pe- 
culiar kind, and exhibit ramifications and junc- 
tions, resembling those of the blood vessels of 
animals. We may also discover, by the aid of 
the microscope, that the fluids contained in these 
vessels are moving in currents with considerable 
rapidity, as appears from the visible motions of 
their globules ; and they present therefore a re- 
markable analogy with the circulation of the 
blood in some of the inferior tribes of animals. 
This curious phenomenon was first observed in 
the Chdidonium by Schultz, in the year 1820; 
add he designated it by the term CyclosiSy in 
order to distinguish it firom a real circulation^ if, 
OD farther inquiry, it should be found not to be 
entitled to the latter appellation.* 

The circular movements, which have been 
thns observed in the milky juices of plants, have 
lately attracted much attention among botanists: 
but considerable doubt still prevails whether these 
appearances afibrd sufficient evidence of the 
existence of a general circulation of nutrient 
juices in the vegetable systems of those plants 
which exhibit them ; for it would appear that in 
reality the observed motions of the fluid, are, in 
every case, partial, and the extent of the circuit 

* '* Die Natur der lebendigen Pflanze." See also Annalei 
des Sciences Naturelles, xxiii, 75. 



very limited. The cause of these motions is not 
yet known j but probably they are ultimately 
referable to a vital contraction of the vessels ; for 
they cease the moment that the plant has re- 
ceived an injury, and are more active in pro- 
portion as the temperature of the atmosphere is 

These phenomena are universally met with in 
all plants that contain milky juices ; but they 
have also been observed in many plants, of which 
the juices are nearly transparent, and contain 
only a few floating globules, such as the Ckara, 
or stone-wort, the Caulinia fragilis, kc* where 
the double currents are beautifully seen under 
the microscope, perfonning a complete circulatitm 
within the spaces of the 
stem that lie beween two 
adjacent knots or joints; 
and wher^ by the pro- 
per adjustment of the 
object, it is easy to see 
at one view both the 
^cending and descend- 
ing streams passing on 
opposite sides of th^ 
stem. Fig. 239 shows 
this circulation in the 
cells of theCauHnia fragilis very highly magnified, 

• Amici, Annales des Sciences Naturelie3,jt. p. 41. 


the diiection of the streams being indicated by 
the arrows. Fig. 240 represents the circulation 
in one' of the jointed hairs, projecting from the 
cuticle of the calyx of the Tradescantia vir- 
gimca^* in each cell of which the same circu- 
latory motion of the fluids is perceptible. 

^ 7. Excretion in Vegetables. 

It had long been conjectured by De CandoUe, 
that the superfluous or noxious particles contained 
in the returning sap are excreted or thrown out by 
the roots. It is evident that if such a process takes 
place, it will readily explain why plants render 
the soil where they have long bisen cultivated 
leas suitable to their continuance in a vigorous 
condition, than the soil in the same spot was origi- 
nally ; and also why plants of a different species 
are firequently found to flourish remarkably well 
in the same situation where this apparent dete- 
rioration of the soil has takeli place. The truth 
of this sagacious conjecture has been established 
in a very satisfactory manner by the recent ex- 
pmments of M. Macaire.t The roots of the 

* Fig. 239 is taken from Amici^'and Fig. 240 from that given 
by Mr. Slack, Trans. See. Arts, rol. xlix. 

t An account of these experiments was &rst published in the 
fifth volume of the '' Memoires de la Soci^te de Physique et 
dUistoire Naturelle de Geneve," and repeated in the " Annates 
des Sciences Naturelles/' xxviii, 402. 


Chandrilla muralis were carefully cleaned,. and 
immersed in filtered rain water : the water was 
changed every two days, and the plant continued 
to flourish, and put forth its blossoms: at the 
€nd of eight days, the water had acquired a 
yellow tinge, and indicated, both by the smell 
and taste, the presence of a bitter narcotic sub- 
stance, analogous to that of opium; a result 
which was farther confirmed by the application 
of chemical tests, and by the reddish brown re- 
siduum obtained from the water by evaporation. 
M. Macaire ascertained that neither the roots 
nor the stems of the same plants, when com- 
pletely detached, and immersed in water, could 
produce this effect, which he therefore concludes 
is the result of an exudation from the roots, con- 
tinually going on while the plant is in a state of 
healthy vegetation. By comparative experiments 
on the quantity of matter thus excreted by the 
roots of the French bean (Phaseolus vulgaris) 
during the night and the day, he found it to be 
much more considerable at night ; an effect 
which it is natural to ascribe to the interruption 
in the action of the leaves when they are deprived 
of light, and when the corresponding absorption 
by the roots is also ^ispended. This was con- 
firmed by the result of some experiments he 
made on the same plants by placing them, during 
day time, in the dark, under which circumstances 
the excretion from the roots was found to be 


immediately much augmented : but, even when 
exposed to the light, there is always some exu- 
dation, though in small quantity, going on from 
the roots. 

That plants are able to free themselves, by 
means of this excretory process, from noxious 
materials, which they may happen to have im- 
bibed through the roots, was also proved by ano- 
ther set of experiments on the Mercurialis annua, 
the Senecio vulgaris, and Srassica campestris, or 
common cabbage. The roots of each specimen, 
after being thoroughly washed and cleaned, were 
separated into two bunches, one of which was 
pot into a diluted solution of acetate of lead, and 
the other into pure water, contained in a sepa- 
rate vessel. After some days, during which the 
plants continued to vegetate tolerably well, the 
water in the latter vessel being examined, was 
found to contain a very perceptible quantity of 
the acetate of lead. The experiment was varied 
by first allowing the plant to remain with its 
roots immersed in a similar solution, and then 
removing it, after careful washing, in order to 
free the roots from any portion of the salt that 
might have adhered to their surface, into a 
vessel with rain water ; after two days, distinct 
traces of the acetate of lead were afforded by 
the water. Similar experiments were made with 
lime-water, and with a solution of common salt, 
instead of the acetate of lead, and were attended 


with the like results. De CandoUe has ascer- 
tainedy that certain maritime plants which yield 
soda, and which flourish in situations very distant 
from the coast, provided they occasionally re- 
ceive breezes from the sea, communicate a saline 
impregnation to the soil in their immediate vi- 
cinity, derived from the salt which they doubt^ 
less had imbibed by the leaves. . 

Although the materials which are thus iexcreted 
by the roots are noxious to the plant which rejects 
them, and would consequently be injurious to 
other individuals of the same species, it does not 
therefore follow that they are incapable of sup- 
plying salutary nourishment to other kinds of 
plants : thus it has been observed that the Soli- 
carta flourishes particularly in the vicinity of the 
willow, and the Oroba7iche, or broom^rape, in 
that of hemp. This fact has also been established 
experimentally by M. IVfacaire, who found that 
the water in which certain plants had been kept 
was noxious to other specimens of the same 
species, while, on the other hand, it produced a 
more luxuriant vegetation in plants of a different 

This fact is of great importance in the theory 
of agriculture, since it perfectly explains the 
advantage derived from a continued rotation of 
different crops in the same field, in increasing 
the productiveness of the soil. It also gives a 
satisfactory explanation of the curious pheno* 


menon oi fahy*ring$y as they are called, that is 
af cirelea of dark green grassy occurring in old 
pafitures i these Dr. Wtdlastoa hM traced to the 
growth > o£ saccessive generations of certain fuhgi^ 
or iiiiislurooms, spreading from, a centrad point.* 
The aeiU which has once contributed to the sup- 
poit . of these fungi, becomes exhausted co: dete- 
riorated with sespeet to the i^ture crops. of the 
same .speeies,,! and the plants,^ therefore, cease 
to be produced cm those spots ; the second year's 
crop consequently appears in the i space of a 
small ring, surrounding the origlliaL centre of 
Tegetation.; and) in every succeeding year, the 
deficieiM:y of nutriment on one side necessarily 
causes the- new roots .to extend themselyes solely 
ia the oppmite direction, and occasions the circle 
of fungi continually to proceed by annual en- 
largement from the centre outwards. An appear- 
ance of luxuriance of Uie grass follows as a na- 
tural consequence; for the soil of aa interior 
ciide will always be enriched and fertilized with 
respect .to the culture of grass, by the decayed 
roots ofi fungiriof the preceding years' growth. 
It oflben happens, indeed, diuing the growth of 
these fungi, that they so completely absorb all 
nutriment from, the soil beneath, that i the her- 
bage, is for a time totally destroyed, giving rise 
to the appearance of a ring bare of grass, sur- 

* Phil. Trans, for 1807, p. 133. 



rounding the dark ring ; but after the fungi have 
ceased to appear^ the soil where they had grown 
becomes darker, and the grass soon vegetates 
^ain with peculiar vigour. When two adjacent 
circles meet, and interfere with each other's pro* 
gress, they not only do not cross each other, but 
both circles are invariably obliterated between 
the points of contact : for the exhaustion occa- 
sioned by each obstructs the progress of the 
other, and both are starved. It would appear 
that different species of fimgi often require the 
same kind of nutriment; for, in cases of the in- 
terference of a circle of mushrooms with another 
of puff-balls, still the circles do not intersect onft 
another, the exhaustion produced by the one 
being equally detrimental to the growth of the 
other, as if it had been occasioned by the pre- 
vious v^etation of its ovm species. 

The only final cause we can assign for the 
series of phenomena constituting the nutritive 
functions of vegetables is the formation of cer- 
tain organic products calculated to supply suste- 
nance to a higher order of beings. The animal 
kingdom is altogether dependent for its support, 
and even existence, on the vegetable world. 
Plants appear formed to bring together a certain 
number of elements derived from the mineral 
kingdom, in order to subject them to the opera- 
tions of vital chemistry, a power too subtle for 
human science to detect, or for human art to 


imitate, and by which these materials are com* 
bined into a variety of nutritive substances. Of 
these substances, so prepared, one portion is con- 
sumed by the plants themselves in maintaining 
their own structures, and in developing the em- 
bryos of those which are to replace them ; another 
portion serves directly as food to various races of 
animals; and the remainder is either employed 
in fertilizing the soil, and preparing it for subse- 
quent and more extended vegetation, or else, 
buried in the bosom of the earth, it forms part of 
that vast magazine of combustible matter, des- 
tined to benefit future communities of mankind, 
when the arts of civiUzation shall have developed 
the mighty energies of human power. 

Chapter III. 


§ 1 . Food of Anhnals. 

Nutrition constitutes no less important a part 
of the animal, than of the vegetable economy. 
Endowed with more energetic powers, and en- 
joying a wider range of action, animals, com- 
pared with plants, require a considerably larger 
supply of nutritive materials for their sustenance, 


and for the exerciae of their various and higher 
faculties. The ixiaterials of animal nutrition 
must, in aU cases, have previously been combined 
in a peculiar mode ; which the .powens of organ- 
ization lalone can effect. In the conversion of 
vegetable into animal matter^ the principal 
changes in chemical composition which the 
former undergoes, are, first,> the abstraction of a 
certain proportion of carbon ; and secondly, the 
addition of nitrogen.* Other changes, however, 
less easily appreciable, though perhaps as im- 
portant as the former^ take place in greater 
quantity, with regard to the proportions of saline 
earthy, and metallic ingredients; all of which, 
and more especially iron, exist in greater quantity 
in animal than in vegetable bodies. The former 
also contain a larger proportion of sulphur and 
phosphorus than the latter. 

The equitable mode in which nature dispenses 
to her innumerable offspring the food she has 
provided, for their subsistence, apportioning to 

* The recent researches of Messrs. Macaire and M arcet tend 
to establish the important fact that both the chyle and the blood 
of herbivorous and of carnivorous quadrupeds are identical in their 
chemical compdsition, in as far, at least, as concerns their ulti- 
mate analysis. They found, in particular, the same proportion 
of nitrogen in the chyle, whatever kind of food the animal habi- 
tually consumed : and it was also the same in the blood, whether 
of carnivorous or herbivorous animals ; although this last fluid 
contains more nitrogen than the chyle. {Mfinoires de la SocieU 
de Physique et d'Histaire Naturelle de Genh)e^ v. 389.) 


each the quantity, and the kind most consoneait 
to enlarged Tiewe.of prospective beneficence, is 
calculated to excite our highest yr<mdeT and 
admiration. While the waste is < the smallest 
possible, we fifid* that nothing which can > afford 
notrimeat is wholly lost. There is no part of the 
organized ^mcture of an animal or regetable, 
bowever dense its texture, or . acrid . its qualities, 
that may not, under certain circumstances, he^ 
come the food of some ^ecie^ of insect, or con- 
tribute, in- some mode to the support .of animal 
life. The more succulent parts of plants, such 
as the leaves, or* softer sterns^ are the principal 
soorces of nourishment to the greater number of 
larger quadrupeds, to multitudes of - insects, as 
well as ta- numerous tribes of other animals. 
Some plants are moire particularly assigned as the 
appropriate mitriment of particular species, which 
would perish if these ceased to grow : thus the 
silkworm subsists almost exclusively upon the 
leaves of the mulberry tree ; and many species of 
caterpillars are attached each to .a particular 
plant which they prefer to all others. There are 
at least fifty different species of insects that feed 
upon the common nettle ; and plants, of which the 
juices are most acrid and. poisonous to the gene- 
TBltty of animals, such ai% Euphorbium, Henbane, 
and Nightshade^ afford a wholesome and deli- 
cious food to others. Innumerable tribes of ani- 
mals subsist upon firuits and seeds ; while others 


feast upon the juices which they extract from 
flowers, or other parts of plants; others, again, 
derive their principal nourishment from the hard 
fibres of the bark or wood. 

Still more general is the consumption of animal 
matter by various animals. Every class has 
its carnivorous tribes, which consume living prey 
of every denomination ; some being formed to 
devour the flesh of the larger species, whether 
quadrupeds, birds, or fish ; others ^ feeding on 
reptiles or moUusca, and some satisfying their 
appetite with insects alone. The habits of the 
more diminutive tribes are not less predatory 
and voracious than those of the larger quad- 
rupeds; for the spiders on the land, and the 
Crustacea in the sea, are but representatives of 
the lions and tigers of the forest, displaying an 
equally ferocious and insatiable rapacity. Other 
families, again, generally of still smaller size, are 
designed for a parasitic existence, their organs 
being fitted only for imbibing the blood or juices 
of other animals. 

No sooner is the signal given, on the death of 
any large animal, than multitudes of every class 
hasten to the spot, eager to partake of the repast 
which nature has prepared. If the carcass be 
not rapidly devoured by rapacious birds, or car- 
nivorous quadrupeds, it never fails to be soon 
attacked by swarms of insects, which speedily 
consume its softer textures, leaving only the 


bones.* These, again, are the favourite repast 
of the Hyaena, whose powerfiil jaws are pecu- 
liarly formed for grinding them into powder, 
and whose stomach can extract from them an 
abundant portion of nutriment. No less speedy 
IS the work of demolition among the inha- 
bitants of the waters, where innumerable fishes, 
Crustacea, annelida, and moUusca are on the 
watch to devour all dead animal matter which 
may come within-their reach. The consumption 
of decayed vegetables is not quite so speedily 
accomplished ; yet these also afford an ample 
store of nourishment to hosts of minuter beings, 
less conspicuous, perhaps, but performing a no 
less important part in the economy of the creation. 
It may be observed that most of the insects which 
feed on decomposing materials, whether animal 
or vegetable, consume a much larger quantity 
than they appear to require for the purposes of 
nutrition. We may hence infer that in their 
formation other ends were contemplated, besides 

* So strongly was Linneeus impressed with the immensity of 
the scale on which these works of demolition by insects are 
carried on in nature, that he used to maintain that the carcass 
of a dead horse would not be devoured with the same celerity 
by a lion, as it would be by three flesh flies (Musca vomitoria) and 
their immediate progeny : for it is known that one female fly will 
give birth to at least 20,000 young larvse, each of which will, in 
the coarse of a day, devour so much food, and grow so rapidly, 
as to acquire an increase of two hundred tunes its weight : and 
a few days are sufficient to the production of a third generation. 


their own iadividual existence. They seem as 
if comraissioned to act as tiie scaveDgeis of organic 
matter, destined to clear away all these particles, 
of whidh the continued accumulation would have 
tainted thcf attnosphere* or the* waters with infec- 
tion, and spread a wide extent of desolation and 
of death. . i . . 

In taking these general surveys of the plans 
adopted by nature for the universa} subsistence 
of the objects of her bounty, we cannot help ad- 
miring how carefully she has provided the means 
for turning to the best account every particle of 
each product of organic life, whether the material 
be consumed as food by animals, or whether it 
be bestowed upon the soil, reappearing in 1^ 
substance of some plant, and being in this way 
made to contribute eventually to the same ulti- 
mate object, namely, the support of animal life; 

But we may carry these views still farther, 
and following* the ulterior destination- of the 
minuter and unheeded fragments of decompooed 
organizations, which we might conceive had been 
cast away, and lost to all useful purposes, we 
may trac6 them as they are swept down by- the 
rains, and deposited in pools and lakes, amidst 
waters collected from the soil on every side. 
Here we find them, under favourable circum- 
stances,' again partaking of animation, and in- 
vested with various forms of infusory animalcules, 


which sport in countless myriads their ephemeral 
existence within the ample regionsof every drop. 
Yet even these are still qualified fo fulfil other 
objects in a more 'distant and far wider sphere; 
for, borne al<»g, in the course of* time, by the 
rivers dnto' which they pass, they are at length 
conveyed into the sea, the great receptacle of all 
the particles that are detached from the objects 
on land. -Here 'also they float not' uselessly in 
the vast abyss, but contribute to maintain in ex- 
istence incalculable hosts of animal beings, which 
people every pK)rtion of »the wide expanse of 
ocean, and* which rise in regular gradation from 
the microscopic monad, and scarcely visible 
medusa,* through endless tribes of moUusca, and 
of fishes, up to the huge Leviathan of the deep; 

Not-evfemtare these portions of i^rganic matter, 
which, inr the course of decomposition, escape in 
the form of gases^ and are widely difiused through 
the atmosphere, ^^diolly lost for the uses of living 
nature': for, in course of time,* they, also, as we 
have seenv re-enter into- the vegetable System, 
resuming > the solid form, and reappearing as 
organic products, destined agsun to run through 

* The immehsity of the numbers of these microscopic medusae, 
which people every region of the ocean, may be judged of from 
the phenomenon of the phosphorescent light which is so fre* 
qoentlj ez}iibited by the sea, when agitated, and which, as I have 
already observed, is found to arise from the presence of an incal- 
culable multitude of these minute animals. 


the same never ending cycle of vicissitudes and 

The diffusion of animals over wide regions of 
the globe is a consequence of the necessity which 
prompts them to search for subsistence wherever 
food is to be met with. Thus while the v^etation 
of each different climate is regulated by the sea- 
sons, herbivorous animals are in the winter forced 
to migrate from the colder to the milder regions^ 
where they may find the pasturage they require ; 
and these migrations occasion corresponding 
movements among the predaceous tribes which 
subsist upon them. Thus are continual inter- 
changes produced, contributing to colonise the 
earth, and extend its animal population over 
every habitable district. But in all these changes 
we may discern the ultimate relation they ever 
bear to the condition of the vegetable world, 
which is placed as an intermediate and necessary 
link between the mineral and the animal king- 
doms. All those regions which are incapable of 
supporting an extensive vegetation, are, on that 
account, unfitted for the habitation of animals. 
Such are the vast continents of ice, which spread 
around the poles ; such are the immense tracts 
of snow and of glaciers, which occupy the sum- 
mits of the highest mountain chains ; and such 
is the wide expanse of sand, which covers the 
largest portions both of Africa and of Asia : and 
often have we heard of the sunken spirits of the 


traveller through the weary desert, from the 
appallmg silence that reigns over those regions of 
eternal desolation ; but no sooner is his eye 
refireshed by the reappearance of vegetation^ 
than he again traces the footsteps and haunts of 
animals, and welcomes the cheering sound of 
sensitive beings. 

The kind of food which nature has assigned 
to each particular race of animals has an impor- 
tant influence, not merely on its internal organ- 
ization, but also on its active powers and dispo- 
sition ; for the faculties of animals, as well as 
their structure, have a dose relation to the cir- 
cumstances connected with their subsistence,* 
sach as the abundance of its supply, the facility 
(^procuring it, the dangers incurred in its search, 
and the opposition to be overcome before it can 
be obtained. In those animals whose food lies 
generally within their reach, the active powers 
acquire but little developement : such, for in- 
stance, is the condition of herbivorous quad- 
nipeds, whose repast is spread every where in 
rich provision beneath their feet ; and it is the 
chief busiufsss of their lives to crop the flowery 
mead,* and repose on the same spot which aflbrds 
them the means of support. Predaceous animals, 
on the contrary, being prompted by the calls of 
appetite to wage war with living beings, are formed 
fer a more active and martial career ; their mus- 
dea are more vigorous, their bones are stronger, 



their limbs more robust, their senses more deli- 
cate and acute. What sight can compare with 
that of the eagle and the lynx ; what scent can 
be more exquisite than that of the wolf and the 
jackall ? All the perceptions of carnivorous ani- 
mals are more accurate, their sagacity embraces 
a greater variety of objects, and in feats of 
strength and agility they far surpass the herbi- 
vorous tribes. A tiger will take a spring of fif- 
teen or twenty feet, and seizing upon a buflQeilo, 
will carry it with ease on its back through a 
dense and tangled thicket : with a single blow 
df its paw it will break the back of a bull, or 
tear open the flanks of an elephant. 

While herbivorous animals are almost con- 
stantly employed in eating, carnivorous animals 
are able to endure abstinence for a great length 
of time, without any apparent diminution of their 
strength : a horse or an ox would sink under 
the exhaustion consequent upon fasting for two 
or three days, whereas the wolf and the martin 
have been known to live fifteen days without 
food, and a single meal will suffice them for a 
whole week. The calls of hunger produce on 
each of these classes of animals the most opposite 
efiects. Herbivorous animals are rendered weak 
and faint by the want of food, but the tiger is 
roused to the full energy of his powers by the 
cravings of appetite ; his strength and courage 
are never so great as when he is nearly famished. 


ted he rushes to the attack, reckless of conse- 
quences, and undismayed by the number or 
force of his opponents. From the time he has 
tasted blood, no education can soften the native 
ferocity of bis disposition : he is neither to be 
reclaimed by kindness, nor subdued by the fear 
(^punishment. On the other hand, the elephant, 
subsisting upon the vegetable productions of the 
forest, superior in size and even in strength to 
the tiger, and armed with as powerAil weapons 
of offence, which it wants not the courage to em- 
ploy when necessary, is capable of being tamed 
with the greatest ease, is readily brought to 
submit to the authority of man, and requites 
with affection the benefits he receives. 

On first contemplating this extensive destruC" 
tioQ of animal life by modes the most cruel and 
revolting to all our feelings, we naturally recoil 
with honor from the sanguinary scene ; and 
cannot refrain £rom asking how all this is consis- 
tent with the wisdom and benevolence so conspi* 
cuoQsly manifested in all other parts of the crea- 
tion. The best theologians have been obliged 
to confess that a difficulty does here exist,* and 
that the only plausible solution which it admits 
of) IS to consider the pain and suffering thus 
created, as one of the necessary consequences of 
thoee general laws which secure, on the whole, 

• See, in particular, Paley*8 Natural Theology, chap, xxvi. 


the greatest and most permanent good. There 
can be no doubt that the scheme, by which one 
animal is made directly conducive to the subsis-: 
tence of another, leads to the extension of 
the benefits of existence to an infinitdy greater 
number of beings than could otherwise have en- 
joyed them. This system, besides, is the spring 
of motion and activity in every part of nature. 
While the pursuit of its prey forms the occupa- 
tion, and constitutes the pleasure of a considerable 
part of the animal creation, the emp]o3aaient of 
the means they possess of defence, of flight, anjd 
of precaution is also the business of a still larger 
part. These means are, in^ a great proportion <^ 
instances, successful ; for wherever nature has in- 
spired sagacity in the perception of danger, she 
has generally bestowed a proportionate degree of 
ingenuity in devising the means of safety. ^ Some 
are taught to deceive the enemy, and to employ 
stratagem where force or swiftness would have 
been unavailing : many insects, when in danger, 
counterfeit death to avoid destruction ; others, 
among the myriapoda, fold themselves into the 
smallest possible compass, so as to escape deted- 
tion. The tortoise, as we have already se^i, 
];etreats within its shell, as within a fortress ; the 
hedge-hog rolls itself into a ball, presenting 
bristles on every side; the diodon inflates its 
globular body for the same purpose, and floats 
on the sea, armed at all the points of its surface ; 


the cuttle-fish screens itself frotn puniuit by effii- 
aiiig an intensely dark coloured ink, which renders 
the surrounding waters so black and turbid as to 
conceal the animal, and favour its escape; the 
torpedo defends itself from molestation by reite- 
rated discharges from its electric battery; the 
buttfeffly avoids capture by its irregular move- 
ments in the air, and the hare puts the hounds 
at fault by her ma;^y doublings. Thus does 
the animated creation present a busy scene 
ef activity and employment : thus are a variety 
of pcrwers called forth, and an infinite diversity 
6f pleasures derived from their exercise; and 
eaustence is on the whole rendered the source of 
incomparably higher degrees, as well as of a larger 
ainoutit of enjoyment, than appears to have been 
esmpatibte with any other imaginable system. 

§ 2. Series of Vital Functions, 

In the animal economy, as in the v^etable, the 
vital, or nutritive functions are divisible into seven 
kinds, namely. Assimilation, Circulation, Respi- 
ration, Secretion, £xcretion, Absorption, and 
Nutrition ; some of which even admit of farther 
snbdivision. This is the cade more particularly 
with the processes of assimilation, which are 
gaierally numerous, and require a very compli- 
citfed apparatus for acting on the food in all the 


stages of its conversion into blood, a fluid which, 
like the returning sap of plants, consists of nutri- 
ment in its completely assimilated state. It will 
be necessary, therefore, to enter into a more par- 
ticular examination of the objects of these diffe- 
rent processes. 

In the more perfect structures belonging to 
the higher orders of animals, contrivances must 
be adopted, and organs provided for seizing the 
appropriate food, and conveying it to the mouth. 
A mechanical apparatus must there be placed 
for effecting that minute subdivision, which is 
necessary to prepare it for the action of the che- 
mical agents to which it is afterwards to be sub- 
jected. From the mouth, after it has been 
sufficiently masticated, and softened by fluid 
secretions prepared by neighbouring glands, the 
food must be conveyed into an interior cavity, 
called the Stomach, where, as in a chemical 
laboratory, it is made to undergo the particular 
change which results from the operation termed 
Digestion. The digested food must thence be 
conducted into other chambers, composing the 
intestinal tube, where it is converted into Chyle^ 
which is a milky fluid, consisting wholly of 
nutritious matter. Vessels are then provided, 
which, like the roots of plants, drink up this 
prepared fluid, and convey it to other cavities, 
capable of imparting to it a powerful impulsive 
force, and of distributing it through appropriate 


channels of circulation, not only to the respi- 
ratory oigans, where its elaboration is completed 
by the influence of atmospheric air, but also to 
all other parts of the system, where such a supply 
is required for their maintenance in the living 
state. The objects of these subsequent functions^ 
many of which are peculiar to animal life, have 
already been detailed.* 

This subdivision of the assimilatory processes 
occurs only in the higher classes of animals, foi" 
in proportion as we descend in the scale, we 
Hod U.em .nore and more simplified, by d.e con- 
eentration of organs, and the union of many offices 
in a single organ, till we arrive, in the very lowrat 
orders, at little more than a simple digestive 
cavity, performing at once the functions of the 
stomach and of the heart ; without any distinct 
circulation of nutrient juices, without vessels, — • 
nay without any apparent blood. Long after 
all the other organs, such as the skeleton, whe- 
ther internal or external, the muscular and ner« 
VOU5 systems, the glands, vessels, and organs of 
sense, have one after another disappeared, we 
still continue to find the digestive cavity retained, 
as if it constituted the most important, and only 
indispensable organ of the whole system. 

The possession of a stomach, then, is the pecu* 
liar characteristic of the animal - system as con- 

* See the first chaptei of this volume, p. 1 1 . 


trasted with that of vegetables. It is a 'distinctiTe 
criterion that applies even to the lowest CKiden 
of SBOophytes, which, in other respects, are so 
nearly allied to plants. It extends to all insects, 
however diminutive ; and even to the minutest 
pf the microscopic animalcules.* 

The mode in which the food is received into 
the body is, in general, very difTerent in the two 
organized kingdoms of nature. Plants receive 
their nourishment by a slow, but nearly constant 
supply, and have no receptacle for collecting it 
at its immediate entry ; the sap, as we have 
seen, passing at once into the cellular tissue of 
the plant, where the process of its gradual elabo* 
ration is commenced. Animals, on the other 
hand, are capable of receiving at once large 
supplies of food, in consequence of having an in-* 
temal cavity, adapted for the immediate recep" 
tion of a considerable quantity. A vegetable 
may be said to bdong to the spot from whic^h it 
imbibes its nourishment, and the surrounding 
soil, into which its absorbing roots are spread 
on every sride, may almost be considered as a 
part of its system. * But an animal has all its 

* In some species of animals belonging to the tribe of mednsee, 
as the Eudora, Berenieey OrytAta, JFVzwmia, Lymnoriaj and 
Geryonia, no central cavity corresponding to a stomach has been 
discovered : they appear, therefore, to constitute an exception to 
the general rule. See Peron, Annales de Museum, xiv, 227 and 


oigans of assimilation within itself, and having 
receptacles in which it can lay in a store of 
pravisions, it may be said to be nourished from 
within ; for it is from these interior receptacles 
that the lacteals, or absorbing yessels, corres* 
ponding in their office to the roots of vege- 
tables, imbibe nourishment. Important conse- 
quences flow from this plan of structure ; for since 
animnlft are thus enabled to subsist for a certain 
imeiral without needing any fresh supply^ they 
iffe independrait of local situation, and may enjoy 
the privilege of moving from ]dace to fdace^ 
Such a power of locomotion was, indeed » abso- 
lutely necessary to beings which have their sob* 
flistence to seek. It is this necessity, again, 
which calls fcMT the continued exercise of their 
flenses, intdligence, and more active energies; 
and that lead, in a word, to the possession of all 
tittse higher powers, which raise them so far 
above the level of the vegetable creation. 

Chapteb IV. 

Ifutrition in the lower Orders of Animals. 

The animals which belong to the order of polypi 
present us with the simplest of all possible forms 
of nutritive oi^ns. The hydra, for instance, 
which may be taken as the type of this formation, 
consists of a mere stomach, provided with the 
simplest instruments for catching food, — and no- 
thing more. A simple sac, or tube, adapted to 
receive and digest food, is the only visiUe organ 
of the body. It exhibits not a trace of either 
brain, nerves, or organs of sense, nor any part 
corresponding to lungs, heart, or even vessels 
of any sort ; all these organs, so essential to 
the maintenance of life in oth» animals, being 
here dispensed with. In the magnified view of 
the hydra, exhibited in Fig. ^1, 
the cavity into which the food is 
^ received and digested is laid open 

by a longitudinal section, so as 
to show the comparative thick- 
ness of the walls of this cavity. 
The structure of th^e walls must 
be adapted not only to prepare 
and pour out the fluids by which the food is di- 
gested, but also to allow of the transudation 


through its substance, probably by means of in* 
vifflble pores, of the nutritious particles thus ex* 
tracted from the food, for the purpose of its being 
incorporated and identified with the gelatinous 
pulp, of which the body appears wholly to consist. 
The thinness and transparency of the walls of 
this cavity allow of our distinctly following these 
changes by the aid of the microscope. Trembley 
watched them with unwearied perseverance for 
days together, and has given the following ac^ 
count of his observations. The hydra, though it 
does not pursue the animals on which it feeds, 
y^ devours with avidity all kinds of living prey 
that come within the reach of its tentacula, and 
which it can overcome and introduce into its 
mouth. The larvee of insects, naides, and other 
aquatic wcnrms, minute Crustacea, and even small 
fishes, are indiscriminately laid hold of, if they 
happen but to touch any part of the long fila* 
ments which the animal spreads out, in difierent 
directions, like a net, in search of food. The 
struggles of the captive, which finds itself en- 
tangled in the folds of these tentacula, are gene- 
rally ineffectual, and the hydra, like the boa 
constrictor, contrives, by enormously expanding 
its mouth, slowly to draw into its cavity ani- 
mals much larger than its own body. Worms 
longer than itself are easily swallowed by being 
jffeviously doubled together by the tentacula* 
Fig, 242 shows a hydra in the act of devouring 


the TomUbmi larva of a Ttpula, which it has 
encircled with its tentacula, to which it has 

"*\*' r« 

ftpplied itd expanded mouth, and of which it is 
absorbing the juice, before swallowing it. Fig. 
243 shows the same animal after it has suc- 
ceeded, though not without a severe contest^ ib 
swallowing a minnow, or other smalt fish, the 
form of which is still visible through the trans- 
parent sides of the body, which are stretched t6 
the utmost. It occasionally happens, when two 
(^ these animals have both seized the same oljett 
by its different ends, that a stru^le between 
t&em ensues, and that the strongest, having ob- 
tained the victory, swallows at a single gulp, not 
only the object of contention, but its antagoniA 
filee. This scene is represented in Fig. 244, 
where the tail of the hydra, of wMch the body 
has been swallowed hj the victor, is seen {ffo- 
tmidiBg from the mouth of the latter. It sooti, 
however, extricates itself from this utuation, 


ipparently without having suffered the smallest 
injury. The voracity of the hydra is very great, 
eqiecially after long fasting ; and it will then 
devour a great number of insects, one after ano- 
ther, at one meal, gorging itself till it can hold 
DO more, and its body becoming dilated to an 
extraordinary size : and yet the same animal can 
continue to live for more than four months with- 
out any visible supply of food. 

On attentively observing the changes, induced 
upon the food by the action of the stomach of 
these animals, they appear to consist of a gradual 
melting down of the softer parts, which are re- 
dived into a kind of jelly, leaving unaltered only 
a few fragments of the harder and less digestible 
poits. These changes are accompanied by a 
kind of uadulation of the contents of the stomach, 
backwards and forwards, throughout the whole 
tube, apparently produced by the contraction 
nod dilatation of its different portions. The un- 
digested materials being collected together and 
iqected by the mouth, the remaining fluid is 
seen to contain opaque globules of various sizes, 
mae of which are observed to penetrate through 
the sides of the stomach, and enter into the gra* 
aukur strnctore which composes the flesh of the 
animal. Some portion of this opaque fluid is 
^tributed to the tentacula, into the tubular 
cavities of which it may be seen entering by 
passages of communication with the stomach. 


By watching attentively the motions of the glo- 
buleSy it will be perceived that they pass back- 
wards and forwards through these passages, like 
ebbing and flowing tides. 

All these phenomena may be observed with 
greater distinctness when the food of the animal 
contains colouring matter, capable of giving a 
tinge to the nutritious fluid, and allowing of its 
progress being traced into the granules which are 
dispersed throughout the substance of the body. 
Trembley is of opinion that these granules are 
vesicular, and that they assume the colour they 
are observed to have, from their becoming filled 
with the coloured particles contained in the nou- 
rishment. The granules which are nearest to 
the cavity of the stomach are those which are 
first tinged, and which therefore first imbibe the 
nutritious juices : the others are coloured succes- 
sively, in an order determined by their distance 
from the surface of the stomach. Trembley 
ascertained that a living hydra introduced into 
the stomach of another hydra, was not in any 
degree acted upon by the fluid secretions of that 
organ, but came out uninjured. It often happens 
that a hydra in its eagerness to transfer its victim 
into its stomach, swallows several of its own ten^* 
tacula, which had encircled it : but these tenta* 
cula always ultimately came out of the stomachy 
sometimes after having remained there twenty* 
four hours, without the least detriment. 


The researches of Trembley have brought U^ 
light the extraordinary fact that not only the 
internal surface of the stomach of the polypus is 
endowed with the power of digesting food, but 
that the same property belongs also to the ex- 
ternal surface, or what we might call the skin of 
the animal. He found that by a dexterous mani- 
pulation, the hydra may be completely turned 
inside out, like the finger of a glove, and that 
the animal, after having undergone this singular 
operation, will very soon resume all its ordinary 
functions, just as if nothing had happened. It 
accommodates itself in the course of a day or 
two to the transformation, and resumes all its 
natural habits, eagerly seizing animalcules with 
its tentacula, and introducing them into its newly 
formed stomach, which has for its interior sur- 
face what before was the exterior skin, and 


which digests them with perfect ease. When the 
discovery of this curious phenomenon was first 
made known to the world, it excited great asto* 
nishment, and many naturalists were incredulous 
as to the correctness of the observations. But 
the researches of Bonnet and of Spallanzani, who 
repeated the experiments of Trembley, have 
borne ample testimony to their accuracy, which 
those of every subsequent observer have farther 
contributed to confirm. 

The experiments of Trembley have also proved 
that every portion of the hydra possesses a won**^ 


derful power of repairing all sorts of injuries, 
and of restoring parts which have beai removed. 
These animals are found to bear with impunity 
all sorts of mutilaticnis. If the tentacula be cut 
off, they grow again in a very short time : the 
whole of the fore part of the body is, in like 
manner, reproduced, if the animal be cut asun- 
der ; and from the head which has been removed 
there soon sprouts forth a new tail. If the head 
of the hydra be divided hy a longitudinal section,, 
extending only half way down the body, the cut 
portions will unite at their edges, so aj» to form 
two heads, each having its separate mouth, and 
set of tentacula. If it be split into six or seven 
parts, it will become a monster with six or seven 
heads ; if each of these be again divided, ano- 
ther will be formed with double that number. 
If any of the parts of this compound polypus be 
cut off, as many new ones will spring up to re- 
^ace them ; the mutilated heads at the same, 
time acquiring fresh bodies, and becoming as 
many entire p<dypi. Fig. 245 represents a hydra 
with seven heads, the result of several operatioi^ 
of this kind. The hydra will sometimes of its 


own accord split into two ; each division be- 
coming independent of the other, and growing 
to the same size as the original hydra. Trembley 
found that different portions of one polype might 
be engrafted on another, by cutting their sur- 
fiEMses, and pressing them together ; for by l^d 


means they quickly unite, and become a com- 
pound animal. When the body of one hydra iB 
introduced into the mouth of another, so that 
their heads are kept in contact for a sufficient 
length of time, they unite and form but one in- 
diyidual. A number of heads and bodies may 
thus be joined together artificially, so as to com-* 
pose living monsters more complicated than the 
wildest fancy has conceived* 

Still more complicated are the forms and eco- 
nomy of those many-headed monsters, which 
prolific nature has spread in countless multitudes 
over the rocky shores of the ocean in every part 
of the globe. These aggregated polypi grow in 
imitation of plants, from a common stem, with 
widely extended flowering branches. Myriads 
of mouths open upon the surface of the animated 
mass; each mouth being surrounded with one 
or more circular rows of tentacula, which are 
extended to catch their prey : but as the station- 
ary condition of these polypes prevents them 
fiom moving in search of food, their tentacula 
aie generally furnished with a multitude of cilia, 
which, by their incessant vibrations, determine 
currents of water to flow towards the mouth, 
carrying with them the floating animalcules on 
which the entire polypus subsists. 

Each mouth leads into a separate stomach ; 
whence the food, after its digestion, passes into 
several channels, generally five in number, which 

VOL* II. o 


proceed in different directions from the cavity of 
each stomachy dividing into many branches, and 
being distributed over all the surromiding portions 
of the flesh. These branches communicate with 
similar channels proceeding from the neigh- 
bouring stomachs : so that the food which has 
been taken in by one of the mouths, contributes 
to the general nourishment of the whole mass of 
aggregated polypi. Cuvier discovered this struc- 
ture in the Veretilla^ which belongs to this order of 
polypi : he also found it in the Pennatula^ and it 
is probably similar in all the others. Fig. 246 
represents three of the polypes of the Veretilla, 
with their communicating vessels seen below. 
The prevailing opinion among naturalists is, that 
each polypus is an individual animal, associated 
with the rest in a sort of republic, where the 
labours of all are exerted for the common benefit 
of the whole society. But it is perhaps more con- 
sonant with our ideas of the nature of vitality to 
consider the extent of the distribution of nutritive 
fluid in any organic system as the criterion of 
the individuality of that system, a view which 
would lead us to consider the entire polypus, or 
mass composed of numerous polypes, as a single 
individual animal ; for there is no more incon- 
sistency in supposing that an individual animal 
may possess any number of mouths, than that it 
may be provided with a multitude of distinct 
stomachs, as we shall presently find is actually 
exemplified in many of the lower animals. 



Some of the Entozoa, or parasitic worms, ex-* 
hibit a general difiusion, or circulation of nou-- 
rishment through numerous channels of commu- 

mcatioiiy into which certain absorbing vessels 
coQYey it from a great number of external orifices, 
or mouths, as they may be called. This is the 
case with the Taniuj or tape worm, which is 
composed of a series of flat jointed portions, of 
^ch two contiguous segments are seen, highly 
magnified, in Fig. 247, exhibiting round the 
margin of each portion, a circle of vessels (v), 
which communicate with those of the adjoining 
segments; each circle being provided with a 
tube (o), having external openings for imbibing 
nourishment from the surrounding fluids. Al- 
though each s^ment 'is thus provided with a 
mtritive apparatus complete within itself, and 
80 fer, therefore, independent of the rest, the 
individuality of the. whole animal is sufficiently 
determined by its having a distinct head at one 


extremity, provided with instruments for its 
attachment to the surfaces it inhabits. 

The Hydatid (Fig. 248) is another parasitic 
worm of the simplest possible construction. It 
has a head (o^ of which h is a magnified repre* 
sentation, furnished with four suckers, and a 
tubular neck, which terminates in a globular 
sac. When this sac, which is the stomach, is 
fully distended with fluid, its sides are stretched, 
so as to be reduced to a very thin transparent 
membrane, having a perfectly spherical shape ; 
after this globe has become swollen to a very 
large size, the neck yields to the distension, and 
disappears; and the head can then be distio* 
guished only as a small point on the surface of 
the globular sac. It is impossible to conceive a 
more simple organic structure than this, which 
may, in fact, be considered as an isolated living 
stomach. The CcenuruSy which is found in the 
brain of sheep, has a structure a little more com- 
plicated ; for instead of a single head, there are 
a great number spread over the surface, opening 
into the same general cavity, and when the sac 
is distended, appearing only as opaque spots on 
its surface. 

The structure of the Sponge has been already 
fully described ; and the course of the minute 
channels pointed out, in which a kind of circu- 
lation of sea water is carried on for the nourish^ 
ment of the animal. The mode by which nutri- 


ment is^xtracted from this circulating fluid, and 
made to contribute to the growth of these plant- 
like structures, is entirely unknown. 

The apparatus for nutrition possessed by 
animals belonging to the tribe of Medusm is of 
a peculiar kind. I have already described the 
iQore ordinary form of these singular animals, 
which resemble a mushroom, from the hemis- 
pherical form of their bodies, and their central 
foot-stalk, or pedicle. In the greater number of 
species there exists at the extremity of this 
pedicle, a single aperture, which is the begin- 
ning of a tube leading into a large central cavity 
in the interior of the body, and which may there- 
fore be regarded as the mouth of the animal: 
but in those species which have no pedicle, as 
the Equoreoy the mouth is situated at the -centre 
of the under surface* Hie aperture is of suffi- 
cient width to admit of the entrance of prey of 
c<»isiderable size, as appears from the circum- 
Mance that fishes of some inches in length are 
occasionally found entire in the stomachs of those 
medusas which have a -single mouth. The central 
tevity, which is t^e stomach of the animal, does 
not appear to possess any proper coats, but to 
be simply scooped out of the soft stnicture of 
the body. Its form varies in different species; 
hating generally, however, more or less of a 
star-like shape, composed of four curved rays, 
which might almost be considered as consti- 


tuting four stomachs, joined at a common ceaitre. 
Such, indeed, is the actual structure in the 
Medusa aurita^ in which Gaede found the 
stomach to consist of four spherical sacs, com« 
pletely separated by partitions. These arched 
cavities, or sacs, taper as they radiate towards 
the circumference, and are continued into a 
canal, from which a great number of other 
canals proceed, generally at first by successive 
bifurcations of the larger trunks, but afterwards 
branching off more irregularly, and again uniting 
by lateral communications so as to compose a 
complicated net-^work of vessels. These rami* 
fications at length unite to form an annular 
vessel, which encircles the margin of the disk. 
It appears also, from the observations of Graede, 
that a farther communication is established 
between this latter vessel, and others which 
permeate the slender filaments, or tentacula, that 
hang like a fringe all round the edge of the 
disk, and which, in the living animal, are in 
perpetual motion. It is supposed that the elon^ 
gations and contractions of these filaments are 
effected by the injection or recession of the 
fluids contained in those vessels."*^ Here, then, 
we see not only a more complex stomach, but 
also the commencement of a vascular system, 
taking its rise from that cavity, and calculated to 

* Journal de Physique, Ixxxix, 146. 


distribute tbe nutritious juices to eyery part of 
the organization. 

There are other species of Medusae, com- 
posing the genus Rkizostoma of Cuvier, which, 
instead of having only one mouth, are provided 
with a great number of tubes which serve that 
office, and which bear a great analogy to the 
loots of a plant.* The pedicle terminates below 
in a great number of fringed processes, which, 
on examination, are found to contain ramified 
tabes, with orifices opening at the extremity of 
each process. In this singular tribe of animals 
there is properly no mouth or central orifice, the 
only avenues to the stomach being ^ese elon- 
gated canals, which collect food from every 
quarter where they extend, and which, uniting 
into larger and larger trunks as they proceed 
towards the body, form one central tube, or 
(Esophagus, which terminates in the general 
cavity of the stomach. The Medusa pulmo, of 
which a figure was given in Vol. i., page 192, 
belongs to this modem genus, and is now termed 
the Rkizostoma Cuvieri. 

The course of these absorbent vessels is most 
conveniently traced after they have been filled 
with a dark coloured liquid. The appearances 
they present in the Rkizostoma Cuvieri, after 

* It is from this circumstance that the genus has received the 
name it now bears, and which is derived from two Greek words, 
^^ing roohlike mouihs. 


being thus injected, are represented in the 
annexed figures; the first of which (Fig. 249), 
shows the under surface of that animal, after the 
pedicle has been removed by a horizontal section, 
at its origin from the hemispherical body, or 

CHpela, as it may be termed, where it has a 
square prismatic form, so that its section presents 
the square surface, o, q. Fig. 252 is a vertical 
section of the same specimen ; both figures being 
reduced to about one-half of the natund size. 
The dotted line, h, h, in the latter figure, shows 
the plane where the section of the pedicle was 
made in order to give the view of the base of the 
hemisphere presented in Fig. 249. On the 
other hand, the dotted line v, v, in Fig. 249, is 
that along which the vertical section of the same 


animal, represented in Fig. 252, was made, four 
oi the anus (a, a, a, a), descending from the 
pedicle, being left attached to it. In these arms, 

or tmtacula, may be seen the canals, marked 
by the dark lines (c, c, c), which arise from 
mnneroas orifices in the extremities and fringed 
nr&ce of the tentacula, and which gradually 
muting, like the roots of a plant, convei^e 
towards the centre of the pedicle, and terminate 
I7 a common tube, which may be considered 
u the oesoi^gus (o), in one large central cavity, 


or Stomach (s), situated in the upper part of the 
cupola. The section of this oesophagus is visible 
at the centre of Fig. 249, where its cavity has 
the form of a cross. The stomach has a quad- 
rangular shape, as in the ordinary medussB, and 
from each of its four comers there proceed 
vessels, which are continuous with its cavity, 
and are distributed by endless ramifications over 
the substance of the cupola, extending even to 
the fringed margin all round its circumference. 
The mode of their distribution, and their nume- 
rous communications by lateral vessels, forming 
a complete vascular net-work, is seen in Fig. 251, 
which represents, on a larger scale, a portion of 
the marginal part of the disk. The two large 
figures (249 and 252) also show the four lateral 
cavities (r, r. Fig. 252), which are contiguous 
to the stomach, but separated firom it by mem- 
branous partitions : these cavities have by some 
been supposed to perform an ofiice in the system 
of the Medusa corresponding to respiration ; an 
opinion, however, which is founded rather on 
analogy than on any direct experimental evi« 
dence. The entrances into these cavities are 
seen open at e, in Fig. 249, and at e, e, in the 
section Fig. 252. A transverse section of one of 
the arms is given in Fig. 253, showing the form 
of the absorbent tube in the centre : and a similar 
section of the extremity of one of the tentacula 
is seen in Fig. 254, in which, besides the central 


tabe» the caYitie9 of some of the smaller hranches 
(b, b), which are proceeding to join it, are also 

The regalar gradation which nature has ob^ 
seired in the onnplexity of the digestive cavities 
and other organs, of the various species of this 
extensive tribe, is exceedingly remarkable : for 
while some, as the Eudofn^ have, to all appear- 
ance, no internal cavity corresponding to a 
stomach, and are totally unprovided with either 
pedicle, arms, or tentacula; others, furnished 
with these latter appendages, are equally desti-* 
tnte of such a cavity ; and those belonging to 
a third family possess a kind of pouch, or false 
stomach, at the upper part of the pedicle, appa- 
rently formed by the mere fojding in of the 
integument. . This is the case with the Gerania^ 
depicted in Fig. 250, whose structure, in this 
respect, approaches that of the Hydra, already 
described, where the stomach consists of an 
open sac apparently formed by the integuments 
alone. Thence may a regular progression be 
followed, through various species, in which the 
aperture of this pouch is more and more com- 
pletely closed, and where the tube which enters 
it branches out into ramifications more or less 
nnmerous, as we have seen in the Rhizostoma.* 
It is difficult to conceive in what mode nutrition 

* See Peron, Annales du Museum, xiv. 330. 


is performed in the agastric tribes, or those 
destitute of any visible stomach ; unless we sup- 
pose that their nourishment is imbibed by direct 
absorption from the surface. 
• Ever since the discovery of the animalcula of 
infusions, naturalists have been extremely de- 
sirous of ascertaining the nature of the organi- 
zation of these curious beings : but as no mode 
presented itself of dissecting objects of such 
extreme minuteness, it was only from the ex- 
ternal appearances they present under the 
microscope that any inferences could be drawn 
with regard to the existence and form of their 
internal organs. In most of the larger species, 
the opaque globules, seen in various parts of the 
interior, were generally supposed to be either 
the ova, or the future young, lodged wi£hin the 
body of the parent. In the Rotifer, or wheel 
animalcule of Spallanzani,* a large central 
organ is plainly perceptible, which was by some 
imagined to be the heart ; but which has been 
clearly ascertained by Bonnet to be a receptacle 
for food. Muller, and several other observers, 
have witnessed the larger animalcules devouring 
the smaller ; and the inference was obvious that, 
in common with all other animals, they also 
must possess a stomach. But as no such struc- 
ture had been rendered visible in the smallest 
species of infusoria, such as monads, it was 

* Vol. i. p. 62, Fio-. I. 


too hastily concluded that these species were 
formed upon a di£ferent and a simpler model* 
Lamark characterized them as being throughout 
of a homogeneous substance, destitute of mouth 
and digestive cayity, and nourished simply by 
means of the absorption of particles through the 
external surface of their bodies. 

The nature and functions of these singular 
beings long remained involved in an obscurity 
which appeared to be impenetrable ; but at 
length a new light has been thrown upon the 
subject by Professor Ehrenberg, whose re- 
searches have recently disclosed fresh scenes of 
interest and of wonder in microscopic worlds^ 
peopled with hosts of animated beings, almost 
infinite in number as in minuteness.* In en- 
deavouring to render the digestive organs of the 
infusoria more conspicuous, he hit upon the for- 
tunate expedient of supplying them with coloured 
fi)od, which might communicate its tinge to the 
cavities into which it passed, and exhibit their 

* The results of Ehrenberg's labours were first communicated 
to the Berlin Academy ; they have since been published in two 
works in German : the first of which appeared at Berlin in 
1830, under the title, of '* Organisation, Systematik und Geo^ 
yrapkisches VerhUltniss der Infusionsthierchen" The second 
vork appeared in 1832, and is entitled '' Zur Erkenntniss der 
Organisation in der Riehtung des kleinsten Raumes,*' Both arq 
in folio, with plates. An able analysis of the contents of the 
fonner of these works, by Dr. Gairdner, is given in The Edin- 
burgh New Philosophical Journal for 1831, p. 201, of which I 
have availed myself largely in the account which follows. 


situation and course. Obvious as this method may 
appear, it was not till after a labour of ten years 
that Ehrenberg succeeded in discovering the 
fittest substances, and in applying them in the 
manner best suited to exhibit the phenomena 
satisfactorily. We l^ve already seen that 
Trembley had adopted the same plan for the 
elucidation of the structure of the hydra.* 
Gleichen also had made similar attempts with 
regard to the infusoria ; but, in consequence of 
his having employed metallic or earthy colour- 
ing materials, which acted as poisons, instead of 
those which might serve as food, he failed in his 
endeavours. Equally unsuccessful were the trials 
made by Ehrenbei^ with the indigo and gum-lac 
of commerce, which are always contaminated 
with a certain quantity of white lead, a sab- 
stance highly deleterious to all animals ; but, at 
length, by employing an indigo which was quite 
pure, he succeeded perfectly.* The moment a 
minute particle of a highly attenuated solution 
of this substance is applied to a drop of water 
in which are some pedunculated vorticellse, oc- 
cupying the field of the microscope, the most 

* The colouring matters proper for these experiments are 
such as do not chemically combine with water, but yet are 
capable of being diffused in a state of very minute division. 
Indigo, sap green, and carmine, answer these conditions, and 
being also easily recognised under the microscopje, are well 
adapted for these observations. Great care should be takeo, 
however, that the substance employed is free from all admixture 
of lead, or other metallic impurity. 


beaQtiAil phenomena present themselves to the 
eye. Currents are excited in all directions by 
the vibrations of the cilia, situated round the 
moQths of these animalcules, and are readily dis- 
dogaished by the motions of the minute particles 
of indigo which are caned along with them; 
the currents generally all converging towards 
the orifice of the mouth. Presently the body 
of the vorticella, which had been hitherto quite 
transparent, becomes dotted with a number of* 
distinctly circular spots, of a dark blue colour, 
evidently produced by particles of indigo accu- 
mulated in those situations. In some species, 
particularly those which have a contracted part, 
OT neck, between the head and the body, as the 
Rotifer vulgaris^ these particles can be traced 
in a continuous line in their progress firom the 
month to these internal cavities. 

In this way, by the employment of colouring 
matters, Ehrenberg succeeded in ascertaining 
the existence of a system of digestive cavities 
in all the known genera of this tribe of animals. 
There is now, therefore, no reason for admitting 
that cuticular absorption of nutritive matter ever 
takes place among this order of beings. Whole 
generations of these transparent gelatinous ani- 
malcules may remain immersed for weeks in an 
lodigo solution, without presenting any coloured 
pcnnls in their tissue, except the circumscribed 
cavities above described. 

Great variety is found to exist in the forms, 


situations, and arrangement of the oi^ans of 
digestion in the Infusoria. They diflfer also in 
their degree of complication, but without any 
obvious relation to the magnitude of the ani- 
malcule. The Monas atomus, the minutest of 
the whole tribe, exhibits a number of sacs, 
opening by as many separate orifices, from a 
circumscribed part of the surface. In others, as 
in the Leucopkra patula, of which Fig. 255 
'represents the appearance under the micro- 

scope, there is a long alimentary canal, tra- 
versing the greater part of the body, taking 
several spiral turns, and furnished with a great 
number of blind pouches, or caca, as sacs of 
this description, proceeding laterally from any 
internal canal, and having no other outlet, are 
technically termed. These cavities become filled 
with coloured particles immediately after their 
entrance into the alimentary canal ; and must 
therefore be considered as so many stomachs 


provided for the digestion of the food which they 
receive.* But they are not all filled at the 
same time, for some continue long in a con- 
tracted state, so as not to be visible ; while, at 
another time, they readily admit the coloured 
food. It is, therefore, only by dint of patient 
watching that the whole extent of the alimentary 
tabe, and its apparatus of stomachs, can be 
My made out. Fig. 255, above referred to, 
exhibits the Leucaphra patula of Ehrenberg,t 
with a few of its stomachs filled with the opaque 
particles: but Fig. 256 shows the whole series 
of (»gans as it would appear if it could be taken 
oot of the body, and placed in the same relative 
ataation with the eye of the observer as they 
are in the first figure. In some species, fi*om 
one to two hundred of these sacs may be 
coanted, connected with the intestinal tube. 
Many of the larger species, as the Hydatina 
teiUa, exhibit a greater concentration of organs, 
haying only a single oval cavity of considerable 
size, situated in the fore part of the body. In 
the Rotifer vulgaris^ the alimentary canal is a 
dender tube, considerably dilated near its termi- 
i^on. In some Vorticellay the intestine, from 
which proceed numerous caeca, makes a complete 
cinndar turn, ending close to its commencement: 

* Ehrenberg tenns these Polygcutric infusoria, from the 
^^'Wk, signifying with many stomachs. 
t Triehoda patula. Mailer. 



Ehrenberg forms of these the tribe of Cydoctsla, 
of which the Vorticella citrina^ and the Sientor 
polymorphns^ are examples. Thus do we dis- 
cover the same diversity in the structure of the 
digestive organs of the several races of these 
diminutive beings, as is found in the other classes 
of animals. 

The Hydati7ia saita, one of the largest of the 
infusoria, was fouhd by Ehrenberg to possess a 
highly developed structure with respect to many 
systems of organs, which we should never have 
expected to meet with so low in the scale of ani- 
mals. As connected with the nutritive functions, 
it may here be mentioned that the head of this 
animalcule is provided with a regular apparatus 
for mastication, consisting of serrated jaws, ieach 
having from two to six teeth. These jaws are 
seen actively opening and shutting when the 
animal is taking its food, which consists of par- 
ticles brought within reach of the mouth by 
means of currents excited by the motions of the 

Such are the simple forms assumed by the 
organs of assfmilation among the lowest orders 
of the animal creation; namely, digesting cavities, 
whence proceed various canals, which form a 
$}'stem for the transmission of the prepared nou- 
rishment to different parts ; but all these cavities 
and canals being simply hollowed out of the 
solid substance of the body. As we ascend a 


Step higher in the scale, we find that the stomach 
and intestinal tube, together with their appen- 
dages, are distinct organs, formed by membranes 
and coats proper to each, and that they are 
themselves contained in an outer cavity, which 
surrounds them, and which receives and collects 
the nutritions juices after their elaboration in 
these organs. The Actinia^ or Sea Anemone^ for 
example, resembles a polypus in its general 
fonn, having a mouth, which isi -Miirounded with 
taitacula, and which leads into > a capacious 
stomach, or sac, open below, and occupying 

the greater part of the 
bulk of the animal ; but 
while, in the poljrpus, 
the sides of the stomach 
constitute also those of 
the body, the whole 
being one simple sac ; in 
the actinia, spaces inter- 
B^ vene between the coats 

of the stomach, and the skin of the animal. As 
the stomach is not a closed sac, but is open below, 
these cavities are, in fact, continuous with that 
of the stomach : they are divided by numerous 
inembranpus partitions passing vertically between 
the skin, and the membrane of the stomach, and 
giving support to that organ. Fig. 257, repre- 
senting a vertical section of the Actinia coriacea^ 
displays this internal structure, b is the base 


or disk, by which the animal adheres to rod^ : 
I is the section of the coriaceous integument, 
showing its thickness : m is the central aperture 
of the upper surface, which performs the office 
of a mouth, leading to s, the stomach, of which 
the lower orifice is open, and which is suspended 
in the general cavity, by means of vertical par- 
titions, of which the cut edges are seen below, 
uniting at a central point, c, and passing between 
the stomach vid the integument. These mus- 
cular partitions are connected above with three 
rows of tentacula, of which the points are seen 
at T. The ovaries (o) are seen attached to the 
partition ; and also the apertures in the lower 
part of the stomach, by which they communicate 
with its cavity. 

If we considered the medusa as having four sto- 
machs, we mighi in like manner regard the Aste- 
rias, or star-fish, as having ten, or even a greater 
number. The mouth of this radiated animal is 

at the centre of the imder surface ; it leads iato 
a capacious bag, situated immediately above U, 


mi which is properly the stomach. From this 
central sac there proceed ten prolongations, or 
canals, which occupy in pairs the centre of 
each ray, or division of the body, and subdiyide 
into numerous minute ramifications. These 
canals, with their branches, are exhibited at c,c, 
F^. 258, which represents one of the rays of the 
Asterias, laid open from the upper side. The 
canals are supported in their positions by mem-^ 
branes, connecting them with the sides of the 
cavity in which they are suspended. 

In the yarious species of Echini^ we find that 
the alimentary tube has attained a more perfect 
^elopement; for instead of constituting merely 
a blind pouch, it passes entirely through the body 
of the animal. We here find an oesophagus , or 
narrow tube, leading from the mouth to the sto- 
mach ; and the stomach continued into a regular 
intestine, which takes two turns in the cayity of 
the body, before it terminates. 

The alimentary tube in the lower animals fre- 
qnently exhibits dilatations in difierent parts; 
these, if situated in the beginning of the cai^al, 
Otty be considered as a succession of stomachs ; 
wifle those that occur in the adyanced portions 
are more properly denominated the great intes- 
tine, by way of distinction from the middle por- 
tions of the tube, which are generally narrower, 
and are termed the small intestine. We often 
flee blind pouches, or cascay projecting from difi*e* 


rent parts of the canal ; this is the case with the 
intestine of the Aphrodita aculeatUy or sea-mouse. 
The intestine, being generally longer than the 
body, is obliged to be folded many times within 
the cavity it occupies, and to take a winding 
course. In some cases, on the other hand, the 
alimentary tube passes in nearly a straight line 
through the body, with scarcely any variation in its 
diameter ; this is the case with the Ascaris, which 
is a long cylindric worm ; and nearly so with the 
Lumbricfis terrestris, or earth-worm. In the Nais, 

on the contrary, as shown 
in Fig. 269, the alimentary 
tube presents a series of 
dilatations, which, from the 
transparency of the skin, 
may be easily seen in the 
living animal. The food taken in by these worms 
is observed to be transferred from the one to the 
other of its numerous stomachs, backwards and 
forwards many times, before its digestion is ac- 

The stomach of the Leech is very peculiar in 
its structure: its form, when dissected off, and 
removed from the body, is shown in Fig. 260. 
It is of great capacity, occupying the larger part 
of the interior of the body ; and its cavity is 
expanded by folds of its internal membrane into 
several pouches (c, c, c). Mr. Newport, who 
has lately examined its structure with great care, 





finds that each of the ten portions into which it 
B divided sends out, on the part most remote 
fiom the cBsophagus (o), two lateral pouches, or 
oeca; which, as they are traced along the 

canal, become both wider and 
longer, so that the tenth pair 
ft 1^^ of caeca (a) extends to the 

**► n **- hinder extremity of the animal; 

the intestine (i), which is very 
short, lying between them.* 
It has long been known, 
that if, after the leech has 
fastened on the skin, a portion 
of the tail be cut off, the ani- 
mal will continue to suck 
blood for an indefinite time: 
this arises from the circum- 
stance that the ceecal portions 
of the stomach are laid open, 
so that the blood received into 
that cavity flows out as fast 
as it is swallowed. 
A structure very similar to that of the leech is 


1 ' 

't "^^ 

* This figure was engraved from a drawing made, at my re- 
quest, by Mr. Newport, from a specimen which he dissected, 
and which he was so obliging as to show me. Fig. 261 repre- 
KQts the mouth, within which are seen the three teeth ; and 
Fig. 262, one of the teeth detached. A paper, descriptive of 
the »tructure of the stomach of the leech, by Mr. Newport, was 
lately read at a meeting of the Royal Society. See the abstracts 
of the proceedings of the Society, for June, 1833. 


met vith in the digestive oigans of the OIosm^ 

pora tubereuUita, (Hirudo complanaita, Linn.) of 

which Fig. 263 rejveseDts 

^^ ^^ a magnified view from.the 

upper side. When Been 

from the under side, as 

i» shown in Fig. 204, the 

cavity of the stomach 

is distinctly seen, pro* 

longed into several cells, 

divided by partitions, and 

directed towards the tail. The two last of these 

cells (c c) are much longer than the rest, and 

terminate in two blind sacs, between which u 

sitiuited a tortuous intiestinal tube.* 

Chapter V. 

Nutritimi in the higher orders of Animals. 

In proportion as we rise in the animal scale, we 
find that the operations of Nutrition beceide 
still farther multiplied, and that the (»^ans which 
perform them are more numerous and more com- 

• In both these figures, t w the tubular tongue, projected 
from the mouth. In Pig. 263, e are the six eyes, situated on 
the extremity which corresponds to the hetid ; uid a doaUe lon- 
gfitudiool row of white tubercles ia also visible, extending along 
the back of the animal, e, id Fig. 264, is the entrance bto a 
cavity, or pouch, provided for the reception of the young. See 
Johnson, Phil. Trans, for 1817, p. 343. 


plicated in their structure. The long series of 
IHOcesses requisite for the perfect elaboration o£ 
ntiiment, is diyided into different stages ; each 
process is the work, of a separate apparatus, and 
requires the influence of different agents. We 
BO longer find that extreme simplicity which we 
noticed as so remarkable in the hydra and the 
aiedusa, whare the same cavity periEbrms at once 
tbe functions of the stomach and of the heart. 
Hie manufacture of nutriment, if we may so 
express it, is, in these lower zoophytes, con- 
ducted upon a small scale, by less refined 
methods, and with the strictest economy of 
means; tbe appa^tus is the simplest, the 
agents the fewest possible, and many dififetent 
operations are carried on in one and the same 

As we follow the extension pf the plan in more 
elevated stages of organic developement, we find 
a farther division of labour introduced. Of this 
we have already seen the commencement in the 
multiplication of the digesting cavities of the 
Leech and other Annelida : but, in animals 
which occupy a still higher rank, we observe 
a more complete separation of offices, and a still 
greater complication of organs. The principle 
of the division of labour is carried to a much 
greater ^ctent than in the inferior departments 
of the animal creation. Besides the stomach, or 
receptacle for the unassimilated food, another 
organ, the heart, is provided for the uniform dis- 


tribution of the nutritious fluide elaborated by 
the organs of digestion. This separation of 
functions, again, leads to the introduction of 
another system of canals ot vessels, for trans- 
mitting the fluids from the organs which prepare 
them to the heart, as into a general reservoir. 
In the higher orders of the animal kingdom, 
all these processes are again subdivided and 
varied, according to the species of food, the 
habits, and mode of life, assigned by nature to 
each individual species. For the purpose of 
conveying clearer notions of the arrangement of 
this extensive system of vital organs, I have 
drawn the annexed plan (Fig. 265), which ex- 

hibits them in their natural order of connexion, 
and as they might be supposed to appear in & 
side view of the interior of a quadruped. To 


this diagram I shaU make frequent reference in 
the following description of this system. 

The food is, in the first place, prepared for 
digestion by several mechanical operations, 
which loosen its texture and destroy its cohe- 
sion. It is torn asunder and broken down by 
the action of the jaws and of the teeth ; and it 
is, at the same time, softened by an admixture 
iritfa the fluid 8ecreti<ms of the mouth. It is 
then collected into a mass, by the action of the 
misdes of the cheek and tongue, and swallowed 
by the regulated contractions of the different 
parts of the throat. It now passes along a mus- 
cular tube, called the (Esophagus, (represented 
in the diagram by the letter o,) into the stomach 
(s), of which the entrance (c) is called the 

•iftthe stomach the food is made to undergo 
IS chemical change's ; after which it is con- 
tiurough the aperture termed the pylorus 
(if^ittto the canal of the intestine (i i), where it 
ilin^er subjected to the action of several fluid 
pkWotts d^ved fix>m large glandular organs 
4Mtod in the neighbourhood, as the liver (l) 
Bifil the pancreas ; and elaborated into the fluid 
which is termed Chyle. 

The Chyle is taken up by a particular set of 
vessels, called the Lacteals, which transmit it to 
the heart (h). These vessels are exceedingly 
numerous, and arise by .open orifices from the 


inner surfSsu^e of the intestines, whence they 
absorb, or drink up the chyle. They may be 
compared to internal roots, which unite as they 
ascend along the mesentery (m), or membrade 
connecting the intestines with the back ; forming 
larger and larger trunks, till they terminate in 
an intermediate reservoir (r), which has beai 
named the Receptacle of the Chyle. From this 
receptacle there proceeds a tube, which, from its 
passing through the thorax, is called the Tho- 
racic duct (t) ; it ascends along the side of the 
spine, which protects it from compression, and 
opens at t, into the large veins which are pour- 
ing their contents into the auricle^ or first cavity 
of the heart (u), whence it immediately passes 
into the ventricle^ or second cavity of that 
organ (<h). Such, in the more perfect animals, 
is the circuitous and guarded route, which every 
particle of nourishment must take before it can 
be added to the general mass of circulating 

By its admixture with the blood already con- 
tained in these vessels, and its purification by 
the action of the air in the respiratory organs (b), 
the chyle becomes assimilated, and is distri- 
buted by the heart through appropriate channels 
of circulation called arteries (of which the com- 
mon trunk, or Aarta^ is seen at a), to every part 
of the system; thence returning by the veins 
(v, r, r,) to the heart. The various modes ia 


winch these functions are conducted in the seve* 
ral tribes of animals will be described hereafter. 
It wiU be sufficient for our present purpose to 
state, by way of completing the outline of this 
class of functions, that, like the returning sap 
of plants, the blood is made to undergo farther 
modifications in the minute vessels through 
which it circulates; new arrangements of its 
elements take place during its passage through 
the subtie organization of the glands, which no 
microscope has yet unravelled: new products 
are here formed, and new properties acquired, 
adapted to the respective purposes which they 
are to serve in the animal economy. The whole 
IS one vast Laboratory, where mechanism is sub- 
servient to Chemistry, where Chemistry is the 
agent of the higher powers of Vitality, and where 
these powers tiiemselves minister to the more 
exalted faculties of Sensation and of Intellect. 

The digestive functions of animals, however 
complex and varied, and however exquisitely 
contrived to answer their particular objects, yet 
afford less favourable opportunities of tracing 
distincdy the adaptation of means to the re- 
spective ends, than the mechanical functions. 
This arises firom the circumstance that the pro- 
cesses they effect imply a refined chemistry, 
q{ which we have as yet but a very imperfect 
knowledge ; and that we are also ignorant of the 
nature of the vital agents concerned in pro- 


ducing each of the chemical changes which the 
food must necessarily undergo during its assimi- 
lation. We only know that all these changes 
are slowly and gradually effected ; the materials 
having to pass through a great number of inter- 
mediate stages before they can attain their final 
state of elaboration. 

Hence we are furnished with a kind of scale 
whereby, whenever we can ascertain the degrees 
of difference existing between the chemical con- 
dition of the substance taken into the body, and 
that of the product derived from it, we may 
estimate the length of the process required, and 
the amount of power necessary for its conversion 
into that product. It is obvious, for example, 
that the chemical changes which vegetable food 
must be made to undergo, in order to assimilate 
it to blood, must be considerably greater than 
those required to convert animal food into the 
same fluid, because the latter is itself darived, 
with only slight modification, immediately fiom 
the blood. We accordingly find it to be an esta- 
blished rule, that the digestive organs of animals 
which feed on vegetable materials are remark- 
able for their size, their length, and their com- 
plication, when compared with those of car- 
nivorous animals of the same class. This rale 
appUes, indeed, universally to Mammalia, Birds, 
Reptiles, Fishes, and also to Insects : and below 
these we can scarcely draw the comparison, 


because nearly all the inferior tribes subsist 
wholly upon animal substances. Many of these 
latter animals have organs capable of extracting 
nourishment from substances which we should 
hardly imagine contained any sensible portion. 
Thus, on examining the stomach of the earth- 
worm, we find it always filled with moist earth, 
which is devoured in large quantities, for the 
sake of the minute portion of vegetable and 
animal materials that happen to be intermixed 
with the soil ; and this slender nutriment is suf- 
ficient for the subsistence of that animal. Many 
marine worms, in like manner, feed apparently 
upon sand alone; but that sand is generally 
intermixed with fragments of shells, which have 
been pulverized by the continual rolling of the 
tide and the surge ; and the animal matter con- 
tained in these firagments, affords them a supply 
of nutriment adequate to their wants. It is evi- 
dent, that when, as in the preceding instances, 
large quantities of indigestible materials are 
taken in along with such as are nutritious, the 
stomach and other digestive cavities must be 
rendered more than usually capacious. It is 
obvious also that the structure of the digestive 
OTgans must bear a relation to the mechanical 
texture, as well as the chemical qualities of the 
food ; and this we find to be the case in a variety 
of instances, which will hereafter be specified. 
The activity of the digestive functions and the 


Structure of the organs, will also be r^ulated by 
a great variety of other circumstances in the 
condition of the animal, independently of the 
mechanical or chemical nature of the food. The 
greater the energy with which the more pecu- 
liarly animal functions of sensation and muscular 
action are exercised, the greater must be the 
demand for nourishment, in order to supply the 
expenditure of vital force created by these exer- 
tions. Compared with the torpid and sluggish 
reptile, the active and vivacious bird or quadruped 
requires and consumes a much larger quantity 
of nutriment. The tortoise, the turtle, the toad, 
the frog, and the chameUon, will, indeed, lire 
for months without taking any food. Fishes, 
which, like reptiles, are cold-blooded animals, 
although at all times exceedingly voracious when 
supplied with food, yet can endure long fasts 
with impunity. 

The rapidity of developement has also great 
influence on the quantity of food which an ani- 
mal requires. Thus the caterpillar, which grows 
very quickly, and must repeatedly throw off its 
integuments, during its continuance in the larva 
state, consumes a vast quantity of food compared 
with the size of its body ; and hence we find it 
provided with a digestive apparatus of consi- 
derable size. 


Chapter VI. 


§ 1 . Prehension of Liquid Food. 

In studying the series of processes which con- 
stitute assimflation, our attention is first to be 
directed to the mode in which the food is in- 
troduced into the body, and to the mechanical 
changes it is made to undergo before it is sub* 
jepted to the chemical action of the digestive 
organs. The nature of these preliminary pro« 
ceases will, of course, vary according to the tex- 
ture and mechanical condition of the food. Where 
it is already in a fluid state, mastication is unne- 
cessary, and the receiving organs consist simply 
of an apparatus for suction. This is the case 
very generally with the Entozoa^ which subsist 
upoif the juices of other animals, and which are all 
provided with one or more sucking orifices, often 
extended in the form of a tube or proboscis.* 
The Hydatid^ for instance, has four sucking 
apertures disposed round the head of the animal : 

* Some species of FascioLe, or flukes, are furnished with two, 
three, six, or more sucking disks, by which they adhere to sur* 
&ces: to these animals the names Distama, Tristoma^ Hexas- 
toma^ and Poly stoma have been given ; but these denominations, 
implying a plurality of mouths, are evidently incorrect, since the 



the Tania has orifices of this kind in each of its 
jointed segments : the Ascaris and the Earth- 
worm hare each a simple mouth. The margia 
of the mouth is often divided, so as to compose 
lips ; of these there are generally two, and in 
the leech there are three. In some rare cases, 
as in the Planaria, there is, besides the ordinary 
mouth, a tube also provided for suction, in a dif- 
ferent part of the body, and leading into the 
same stomach.* 

Wben the instrument for suction extends for 
some length from the mouth, it is generally termed 
^.proboscis: such is the apparatus of the butterfly, 
the moth, the gnat, the house fly, and other 
insects that subsist on fluid aliment. The pro- 
boscis of the Lepidoptera, (Fig. 266), is a double 
tube, constructed by the two 
edges being rolled lon^tudi- 
nally till they meet in the 
middle of the lower surface, 
thus forming a tube on each 
side, but leaving also another 
tube, intermediate to the two 
lateral ones. This middle 
tube is formed by the junction 

■nckiDg; disks are not perforated, and do not perform the office 
of moutho ; and the true mouth for the reception of food is single. 
Cuvier discovered an animal of this class furnished with above a 
hundred of these cup-shaped sucking oi^ns. See Edinbuij:fa 
PhUos. Journal. xi£. 101. 

• Phil Traui. for 182-2, 442. 


of two grooves, which, by the aid of a curious 
apparatus of hooks, resembling those of the la-^ 
minae of a feather abeady described,* lock into 
each other, and can be either united into an air 
tight canal, or be instantly separated at the 
I^eimire of the animal. Reaumur conceives that 
the lateral tubes are intended for the reception 
of air, while the central canal is that which con- 
veys the honey, which the insect sucks from 
flowers, by suddenly imrolling the spiral coil, 
into which the prcrfioscis is usually folded, and 
ifatfting it into the tiectaiy«t 

In the Hemiptera^ the proboscis is a tube, 
either straight or jointed, guarded by a sheath, 
and acting like a pump. The Diptera have a 
more complicated instrument for suction, con^ 
sisting of a tube, of which the sides are strong 
and fleshy, and moveable in every direction, 
like the trunk of an elephant : it has, at its ex^ 
tremity, a double fold, resembling lips, which 
are well adapted for suction. The gnat, and 
other insects which pierce the i^in of animals, 
hare, for this purpose, instruments termed, from 
dieir shape and office, lancets.X In the gnat they 
are five or six in number, finer than a hair, ex- 
ceedingly sharp, and generally barbed on one 
side* In the TabanuSy or horse-fly, they are flat 


• Volume i. page 570. 

t Kirby and Spence*s Entomology, vol. ii. p. 390. 

X Ibid, vol. iii. p. 467. 


like the blade of a knife. These instruments 
are sometimes constructed so as to form, by their 
union, a tube adapted for suction* In the flesh- 
fly, the proboscis is folded like the letter Z, the 
upper angle pointing to the breast, and the lower 
one to the mouth. In other flies there is a single 
fold only. 

Those insects of the order Hymenoptera, 
which, like the bee, suck the honey of flowers, 
have, together with regular jaws, a proboscis 
formed by the prolongation of the lower lip, 
which is folded so as to constitute a tube : this 
tube is protected by the mandibles, and is pro- 
jected forwards by being carried on a pedicle, 
which can be folded back when the tube is not 
in use. The mouths of the Acephalous MoUusca 
are merely sucking apertures, with folds like 
lips, and without either jaws, tongue, or teeth, 
but having often tentacula arising from their 

Among fishes, we meet with the family of 
Cyclostamataj so called from their having a cir- 
cular mouth, formed for suction. The margin 
of this mouth is supported by a ring of cartilage, 
and is furnished with appropriate muscles for 
producing adhesion to the surfaces to which it i^ 
applied ; the mechanism and mode of its attach* 
ment being similar to that of the leech. To this 
family belong the Myxine and the Lamprey. 
So great is the force of adhesion exerted by this 


socking apparatus, that a lamprey has been 
raised out of the water with a stone, weighing 
ten or twelve pounds, adhering to its mouth. 

Hamming birds have a long and slender tongue, 
which can assume the tubular form, like that of 
the butterfly or the bee, and for a similar pur- 
pose, namely, sucking the juices of flowers. 
Among the mammalia, the Vampire Bat affords 
another instance of suction by means of the tongue, 
which is said to be folded into a tubular shape 
for that purpose. But suction among the mam- 
malia is almost always performed by the muscles 
of the lips and cheeks, aided by the movements 
of the tongue, which, when withdrawn to the 
back of the cavity, acts like the piston of a 
pmoip. In the lamprey this hydraulic action 
of the tongue is particularly remarkable. Many 
quadrupeds, however, drink by repeatedly dip- 
ping their tongue into .the fluid, and quickly 
drawing it into the mouth. 

§ 2. Prehension of Solid Food. 

When the food consists of solid substances, 
organs must be provided; first, for their pre- 
hension and introduction into the mouth ; se- 
condly, for their detention when so introduced ; 
and thirdly, for their mechanical division into 
smaller fragments. 


' Of those instruments of prehension which are 
not portions of the mouth itself, and which form 
a serilBS of variously constructed organs, extend- 


ing from the tentacula of the polypus to the 
proboscis of the elephant, and to the human 
arm and hand, some account has already been 
given in the history of the mechanical ftmctions: 
but, in a great number of instances, prehension 
is performed by the mouth, or the parts which 
are extended from it, and may be considered as 
its appendices. The prehensile power of the 
mouth is derived principally from the mecha- 
nical form and action of the jaws, which opai to 
receive, and close to detain the bodies intended 
as food; and to this latter purpose, the teedi, 
when the mouth Is frirnished with them, likewise 
Aiaterially contribute, although their primary 
and more usual office is the mechanical division 
of the food, by means, of mastication, an action 
in which the jaws, in their turn, co-operate. 
Another principal purpose effected by the jaws 
is that of giving mechanical power to the 
muscles, which, by acting upon the sides of the 
cavity of the mouth, tend to compress and 
propel the contained food. We find, accord- 
ingly, that all animals of a highly developed 
structure are provided with jaws. 

Among the animals which are ranked in the 
class of Zoophy t^es, the highest degrees of deve- 
lopement are exhibited by the Echinodermata, 


afid io them we find a remarkable perfection in 
the organs of mastication. The mouth of the 
Eeiimis is surrounded by a frame-work of shell, 
conaistiBg of five converging pieces, each armed 
with a long tooth ; and for thj^ movement of 
each part there are provided separate muscles, 
of which the anatomy has been minutely de- 
scribed by Cuvier. In the shells of the echini 
that are cast on the shore, this calcareous frame 
is usually found entire in the inside of the outer 
case; and Aristotle having noticed its resem- 
Uanoe to a lantern, it has often gone by the 
whimsical name of 'the lantern of Aristotle. 

In all articulated animals which subsist on 
solid aliment, the apparatus for the prehension 
and mastication of the food, situated in the 
moirth, is . exceedingly complicated, and admits 
of great diversity in the different tribes ; and, 
indeed, the number and variety of the parts of 
wUch it consists is so great, as hardly to admit 
of being comprehended in any general descrip- 
tion. In most insects, also, their minuteness is 
an additional obstacle to the accurate obser- 
^tion of their anatomy, and of the mechanism 
of their action. The researches, however, of 
Savigny* and other modem entomologists have 
goae far to prove, that amidst the infinite vari- 

* See his " Theorie des Organes de la bouche des Animaux 
mfert^brfes et articulfa/' which fonns the first part of the ** Me- 
voiles sur les AnimauiL sans verl^res/' Paris, 1816, 


ations observable in the form and arrangement 
of the several parts of these organs, there is still 
preserved, in the general plan of their con- 
struction, a degree of uniformity quite as great 
as that which has been remarked in the fabric 
of the vertebrated classes. Not only may we 
recognise in every instance the same elements 
of structure, but we may also trace r^ular 
chains of gradation connecting forms apparently 
most remote, and organs destined for widely dif- 
ferent uses : so that even when there has been 
a complete change of purpose, we still perceive 
the same design followed, the same model 
copied, and the same uniformity of plan pre- 
served in the construction of the organs of every 
kind of mastication ; and there prevails in them 
the same unity of system as is displayed in so 
marked a manner in the conformation of the 
organs of progressive motion. The jaws, which 
in one tribe of insects are formed for breaking 
down and grinding the harder kinds of food, 
are, in another, fitted for tearing asunder the 
more tough and fibrous textures ; they are 
fashioned, in a third, into instruments for taking 
up the semi-fluid honey prepared by flowers; 
while, again, in a fourth, they are prolonged 
and folded into a tubular proboscis, capable of 
suction, and adapted to the drinking of fluid 
aliment. Pursuing the examination of these 
organs in another series of articulated animals. 


we find them gradually assuming the characters, 
as well as the uses of instruments of prehension, 
of weapons for warfare, of pillars for support, of 
levers for motion, or of limbs for quick pro- 
gression. Some of these remarkable metamor* 
pboses of organs have already attracted our 
attention in a former part of this treatise.* Jaws 
pass into feet, and feet into jaws, through every 
intermediate form ; and the same individual 
often exhibits several steps of these transitions ; 
and is sometimes provided also with super- 
numerary organs of each description. In the 
Arachnida, in particular, we frequently meet with 
supernumerary jaws, together with various ap- 
pendices, which present remarkable analogies of 
form with the antennse, and the legs and feet of 
the Crustacea^ 

The principal elementary parts which enter 
into the composition of the mouth of an insect, 
wfafen in its most perfect state of developement, 
are the seven following : a pair of upper jaws, 
a pair of lower jaws, an upper and a lower lip, 
aiid a tongue.^ These parts in the Locusta 

• Vol. i. p. 289. 

t AH these parts, taken together, were termed by Fabricius 
imstrumenta Maria ; and upon their varieties of structure he 
foonded bis celebrated system of entomological classification. 
Kiiby and Spence have denominated them trophic See their 
Introduction to Entomology, vol. iii. p. 417. To the seven 
elements above enumerated Savigny adds, in the HemipterOy an 
eighth, which he terms the Epiglossa. 


viridissima, or cotnmtm gvasahopper, are deli- 
Deated la their relative skufitiona, but d^ached 
fn>m<one another, in Fig. 267. The upper jaws 
(h), which are termed the mandibles, are thoee 

principally employed for the mastication of hard 
substances ; they are accordingly of greater 
strength than the lom& jawB^ and their edges 
are generally deeply serrated, so as' to act like 
teeth in dividing and bnrisiag the food. Some of 
these teeth are ptiiated, others wedge-ahaped, and 
others broad, like grinders ; their form being in 
each particular case adapted to the mechanical 
texture of the substances to which they are 
designed to be applied. Thus the mandibles 
of some MelohntheB have a projection, rendered 
rough by numerous deep transverse furrows, 
converting it into a file for wearing down the 


diy leaves which cure their natural food.* In 
most cases, indeed, we are, in like manner, 
enabled, from a simple inspection of the shape 
rf (he teeth, to form tolerably accurate ideas of 
the kind of food on which the insect najtuirally 


Above, or rather in front of the mandibles, is 
sitoated the labrunij or upper lip (u). It is 
Qsoally of Q hard or horny texture, and admits 
of some degree of moticm : but its form and 
direction are exceedingly various in different 
tribes of insects. The lower pair of jaws (j), or 
numtUe^ as they have been termed, are behind 
the mandibles, and between them is situated the 
lalrium^ or lower lip (l), which closes the mouth 
bdow, as the Idbrum does abovei In the grass* 
hopper, each maxilla consists of an outer and 
an inner plate (6 and i). The jaws of insects 
are confined, by their articulations .with jthe 
head, to motions; in a horizooital plane only, so 
thai they open and close by. a lateral moyement, 
antLnot vertically upwards; and downwards, as is 
the ease with.thie jawB of vertebrated animajis. 
The maxillse are, in most cases, employed prin- 
cipally for holding the substances on which the 
dinding or grinding apparatus of the mandibles 

* Riioch, quoted by Kirby. 

t See a memoir by Marcel des Serres, in the Annales du 
Vusium d'Hist* Nat. x\v. 56. 


is exerted. A similar use may be assigned also 
to the organs denominated Palpi^ or AnlennuUe 
(p, q), which are jointed filaments, or processes, 
attached to different parts of the mouth, and 
most usually to the maxillsB and the labium; 
the former (p) being termed the maxillary ^ and 
the latter (q) the labial palpi. In addition to 
these parts, another, which, from its supposed 
use, has been denominated Glossay or tongue 
(g), is also generally found. 

For an account of the various modifications 
which these parts receive in different tribes and 
species, I must refer to works which treat pro- 
fessedly of this branch of comparative anatomy. 
I shall content myself with^ giving a single 
example of the conversion of structure here 
alluded to, in that of the rastrunij or proboscis of 
the Cimex nigricornis. This insect belongs to 
the order Hemiptera, which has been usually 
characterised as being destitute of both man- 
dibles and jaws, and as having, instead of these 
parts, an apparatus of very different construe* 
tion, designed to pierce the skin of animals and 
suck their juices. But Savigny, on applying 
the principles of his theory, has recognised, in 
the proboscis of the Cimex, the existence of all 
the constituent elements that are found in the 
mouth of insects formed for the mastication of 
solid food. This proboscis consists of four don- 
gated filaments, contained in a kind of sheath : 



these filaments are represented in. Fig. 268, 

separated to a little distance 
from each other, in order that 
their respective origins may 
be distinctly seen ; the one 
set (q) being prolongations of 
the mandibles^ (j), and the 
other set (p) being, in like 
manner, prolongations of the 
maxillae (m). Between these 
filaments, and near their com- 
mencement, is seen a pointed 
cartilaginous body (g), which 
is the glossa, or tongue ; and 
the aperture seen at its root is 
the passage into the oesopha- 
gus. The sheath is merely 
the elongated labium, of which 
the base is seen at l, in Fig. 
268; but is represented in its whole length in 
Fig, 269, where the groove for containing the 
filaments above described, is apparent. 

In the mouths of the Annelida we often meet 
with hard bodies, which serve the purposes of 
jaws and of teeth. The retractile proboscis of 
the Aphrodite^ or sea-mouse, is furnished with 
four teeth of this description. The Leech has, 
immediately within its lips, three semi-circular 
teeth, with round and sharp cutting edges : they 
are delineated .in Fig. 262, in their relative 



positions; and Fig. 263 represents one of the 
teeth detached from the rest. It is with these 
teeth that the leech pierces the skin of the 
animals whose blood it sucks ; and as soon as 
the wound is inflicted, the teeth, being moveable 
at their base, fall back, leaving the opening of 
the mouth free for sucking. The wound thus 
made is of a peculiar form, being composed of 
three lines, radiating from a centre, where the 
three teeth had penetrated. 

Most of the Mollusca which inhabit univalve 
shells are provided with a tubular organ, of a 
cylindric or conical shape, capable of elongation 
and contraction, by circular and longitudinal 
muscular fibres, and serving the purpose of a ^ 
proboscis, or organ of prehension for seizing and 
conveying food into the mouth. These tubes 
are of great size in the Buccinum^ the Murex^ 
and the VolutUj as also in the Doi^^ which, 
though it has no shell, is likewise a gasteropode. 
In those mollusca of this order which have not a 
proboscis, as the Limaxj or slug, the Hdix^ or 
snail, and the Aplysia^ or sea-hare, the mouth 
is fiimished with broad lips, and is supported by 
an internal cartilage, having several tooth-like 

270 projections, which assist in laying hdd 
^Mjk of the substances taken as food. That 
•^ of the snail is represented in Fig. 270. 

All the Sepi€B, or cuttle fish tribe, are for- 
nished, at the entrance of the mouth, with two 


horny jaws, having a remarkable resemblance 
to the bill. of a parrot; excepting that liie lower 
piece is the largest of the two, and eovers the 
upper one, which is the reverse of what takes 
place in the parrot. These constitute a powerftil 
JAstrament for breaking tlie shells of the mol* 
hisca and Crustacea which compose the usual 
prey of these animals^ 

Fishes almost always swallow their food entire^ 
so that their jaws and teeth are employed prin- 
cipally as organs of prehension and detention ; 
and the upper jaw, as well as the lower one, 
being moveable upon the cranium, they are 
capable of opening to a great width. The bony 
pieces which compose the jaws are more nume-r 
nms than the corresponding bones in the higher 
daases of yertebrata, and they appear, therefore, 
as if their developement had not proceeded suf- 
ficiendy far to ^ect their consolidation into 
more compact structures.* 

Fishes which live upon other animtils of the 
same class having a soft texture, are furnished 
with teeth constructed merely for seizing their 
prey, and perhaps also for slightly dividing it, 
80 as to adapt it to being swallowed. These 
teeth are of various shapes, though usually sharp 

* Attempts have been made to trace analogies between the 
different segments of the jaws of fishes and corresponding parts 
of the mouths of Crustacea and of insects : but the justness of 
t^ analogies is yet far from being satisfactorily proved. 


at the points, and either conical or hooked at 
the extremity, with the points always directed 
backwards, in order to prevent the escape of the 
animal which has been seized. Those fishes 
which subsist on testaceous mollusca have teeth 
with grinding surfaces, and their jaws are also 
adapted for mastication. Every part of the 
mouth, tongue, and even throat, may afford 
lodgement for teeth in this class of animals. 
Almost the whole cavity of the mouth of the 
Anarrhichas lupus^ or wolf-fish, may be said 
to be paved with teeth, a triple row being im- 
planted on each side; so that this fish exerts 
great power in breaking shells. The Shark has 
numerous rows of sharp teeth, with serrated 
margins: these at first sight appear to be for- 
midable instruments; but as the teeth in the 
opposite jaws do not meet, it is evident that they 
are not intended for cutting, like the incisors of 

Amon^ Reptiles, we find the Batrachia almost 
wholly destitute of teeth. Frogs, indeed, exhibit 
two rows of very fine points ; the one in. the 
upper jaw, and the other passing transversely 
across the palate: they may be considered as 
teeth existing in a rudimental state ; for they 
are not sufficiently developed to be useful in 
mastication. There are about forty of these 
minute teeth on each side in the frog. In the 
Salamander, there are sixty above and below; 
and also thirty on each side of the palate. 


The tongue of the frog is of great length ; its 
root is attached close to the fore part of the 
lower jaw, while its point, which is cloyen, is 
turned backwards, extending into the throat and 
acting like a valve in closing the air passage 
into the lungs. If, when this animal has ap- 
proached within a certain distance of the insect 
it is about to seize, we watch it with attention, 
we are surprised to observe the insect suddenly 
disappear, without our being able to perceive 
what has become of it« This arises from the 
firog having darted out its tongue upon its victim 
with such extreme quickness, and withdrawn it, 
with the ii^ect adhering to it, so rapidly, that it 
is scarcely possible for the eye to follow it in its 
motion. The Chameleo?i also has a very long 
and slender tongue, the extremity of which is 
dilated into a kind of club, or spoon, and covered 
with a glutinous matter: with this instrument 
the animal catches insects from a considerable 
distance, by a similar manoeuvre to that prac* 
tised by the frog.* 

Serpents and Lizards have generally curved 
or conical teeth, calculated rather for tearing and 
holding the food, than for masticating it: like 
those of fishes, they are afiixed partly to the 

* Mr. Houston has given a description of the structure of this 
organ, and of the muscles by which it is moved, in a paper con 
Uined in the Transactions of the Ro^al Irish Academy, vol. xv. 
p. 177. 



jaws, and partly to the palate. The Chelouian 
reptiles have no teeth ; their office being sup- 
plied by the sharp cutting edges of the homy 
portion of the jaws. 

Birds, as well as serpents, have a moveable 
upper jaw ; but they are also provided with 
beaks of various forms, in which we may trace 
an exact adaptation to the kind of food appro- 
priated to each tribe : thus predaceous birds, as 
the eagle and the hawk tribe, have an exceed- 
ingly strong hooked beak, for tearing and di- 
viding the flesh of the animals on which they 
prey; while those that feed on insects, or on 
grain, have pointed bills, adapted to lacking up 
minute objects. Aquatic birds have generally 
flattened bills, by which they can best select 
their food among the sand, the mud, or the 
weeds at the bottom of the water; and their 
edges are frequently serrated, to allow the fluid 
to filter through, while the solid portions are 
retained in the mouth. The duck afibrds an 
instance of this structure; which is, however, 
still more strongly marked in the genus Mergus, 
or Merganser, where the whole length of the 
margin of the bill is beset with small sharp 
pointed teeth, directed backwards : they are par- 
ticularly conspicuous in the Mergus serrator^ or 
red-breasteid Merganser. The object of the 
barbs and fringed processes which are appended 
to the tongue in many birds, such as that of the 


Tauean and the Parrakeet, appears, in like 
manner, to be the detention of substances intro* 
daoed into the mouth. 

The beak of the Htematopus^ or Oyster*catcher, 
has a wedge shape, and acts like an oyster- 
knife for opening biyalve shells. 

In the Zjoxia curvirostra, or Cross-bill, the 
upper and lower mandibles cross each other 
idien the mouth is closed, a structure which 
enables this bird to tear open the cones of the 
pine and fir, and pick out the seeds, by insi- 
nnating the bill between the scales. It can split 
cherry stoned with the utmost ease, and in a 
rery short time, by means of this peculiarly 
shaped biU.* 

Birds which dive for the puirpose of catching 
fish haye often a bill of considerable length, 
which enables them to secure their prey, and 
change its position till it is adapted for swal- 

The Rhynchaps^ or black Skimmer, has a very 
singularly formed beak ; it is very slender, but 
the lower mandible very much exceeds in length 
the upper one, so that while skimming the 
waves in its flight, it cuts the water like a 
plough-share, catehing the prey which is on the 
surfece of the sea. 

The Woodpecker is furnished with a singular 

* See a paper on the mechanism of the bill of this bird, by 
Mr. Yarrell, in the Zoological Journal, iv. 459. 


apparatus for enabling it to dart out with great 
velocity its l<mg and pointed tongue, and transfix 
the insects on which it principally feeds; and 
these motions are performed so quickly that the 
eye can scarcely follow them. This remarkable 
mechanism is delineated in Fig. 271, which 
represents the head of the woodpecker, with the 
skin removed, and the parts dissected. The 
tongue itself (t) is a slender sharp-pointed 
homy cylinder, having its extremity (b) beset 
with barbs, of which the points are directed 
backwards: it is supported on a slender Os 
Htfoides, or lingual bone, to the posterior end 
of which the extremities of two very long and 
narrow cartilaginous processes are articulated.* 
The one on the right side is shown in the figure, 

* These cartilages correspond in situation, at the part, at 
least, where they are joined to the os hyoides, to what are called 
the comua, or horns of that bone, in other animals. 


nearly in the whole extent of its coCirse» at c^ d, 
E, F, and a small portion of the left cartilage is 
seen at l. The two cartilages form, at their 
junction with the tongue, a very acute angle, 
slightly diverging as they proceed backwards; 
until, bending downwards (at c), they pass ob- 
liquely round the sides of the neck, connected 
by a membrane (m) ; then, being again inflected 
upwards, they converge towards the back of the 
head, where they meet, and, being enclosed in a 
common sheath, are conducted together along a 
groove, which extends forwards, along the middle 
line of the cranium (e), till it arrives between 
the eyes. From this point, the groove and the 
two cartilages it contains, which are now more 
closely conjoined, are deflected towards the 
right side, and terminate at the edge of the 
aperture of the right nostril (f), into which the 
united cartilages are finally inserted. In order 
that their course may be seen more distinctly, 
these cartilages are represented in the figure 
(at i>), drawn out of the groove provided to 
receive and protect them.* A long and slender 
muscle is attached to the inner margin of each 
of these cartilages, and their actions conspire to 
raise the lower and most bent parts of the 
cartilages, so that their curvature is diminished, 
and the tongue protruded to a considerable dis- 

* S ill the lari^e salivary gland on the right side. 


tance, for the purpose of catching insectd. As 
scion as this has been accomplished, these 
muscles being suddenly relaxed, another s^ of 
fibres, passing in front of the anterior portion of 
the cartilages nearly parallel to them, are thrown 
into action, and as suddenly retract the tongue 
into the mouth, with the insect adhering to its 
barbed extremity. This muscular efibrt is, how- 
ever, very materially assisted by the long and 
tortuous course of these arched cartilages, which 
are nearly as elastic as steel springs, and effect 
a considerable saving of muscular power.* This 
was the more necessary, because, while the Inrd 
is on the tree, it repeats these motions almost 
incessantly, boring holes in the bark, and pick- 
ing up the minutest insects, with the utmost 
celerity and precision. On meeting with an ant- 
hill, the woodpecker easily lays it open by the 
combined efforts of its feet and bill, and soon 
makes a plentiful meal of the ants and their 


Among the Mammalia which have no teeth, 
the Myrmecophagay or Ant-eater, practises a re- 
markable manoeuvre for catching its prey. The 
tongue of this animal i^ very long and slender, 
and has a great resemblance to an earth-worm : 
that of the two-toed ant-eater is very nearly 
one-third of the length of the whole body ; and 

* An account of this mechanism is given by Mr. Waller, in 
the Phil. Trans, for 1716, p. 509. 


at ite base is scarcely thickcsr than a crow-quill. 
It is furnished with a long and powerAil muscle^ 
which arises from the sternum, and is continued 
into its substance, affording the means of a quick 
retraction, as well as lateral motion; while its 
ebngation and other movements are effect^ by 
drcular fibres, which are exterior to the former. 
When laid on |he ground in the us^al track; of 
ants, it is soon covered with these insects, and 
bebg suddenly retracted, transfers them into 
the mouth ; and as, from their minuteness, they 
require no mastication, they are swallowed un- 
divided, and without there being any necessity 
for teeth. 

The lips of quadrupeds are often, elongated for 
the more ready prehension of food, as . we see 
exemplified in the Rhinoceros^ whose upper lip 
is so extensible as to be capable of performing 
^ office of a smfdl proboscis. The Sorw 
moscJiatus, or musk shrew, whose favourite fotod 
18 leeches, has likewise a very moveable SQOut, 
by which it gropes for, and seizes its prey from 
the bottom of the mud. More frequently, how- 
ever, this, office of prehension is performed by 
the tongue, which for that purpose is very 
flexible and much elqi^ated> as we see in the 
Cameleopard, where it acts like a. \k^nji in graspr 
iog and bringing down the branches of a 

* Home, Lectures, &c. vi. Plate 32. 


In the animals belonging to the genus FelU^ 
each of the papillae of the tcmgue is armed with 
a homy sheath terminating in a sharp point, 
which is directed backwards, so as to detain the 
food and prevent its escape. These prickles are 
of great size and strength in the latger beasts of 
prey, as the Lion and the Tiger ; they are met 
with also in the Opossum, and in many species 
of bats, more especially those belonging to the 
genus Pteropus: all these horny productions 
have been regarded as analogous to the lingual 
teeth of fishes, already noticed. 

The mouth of the Omithorkyncus has a form 
of construction intermediate between that of 
quadrupeds and birds ; being furnished, like 
the former, with grinding teeth at the posterior 
part of both the upper and lower jaws, but 
they are of a homy substance ; and the mouth 
is terminated in front by a homy bill, greatly 
resembling that of the duck, or the spoon- 

The Whale is furnished with a singular appa- 
ratus designed for filtration on a large scale. 
The palate has the form of a concave dome, and 
from its sides there descends vertically into the 
mouth, a multitude of thin plates set parallel to 
each other, with one of their edges directed 
towards the circumference, and the other towards 
the middle of the palate. These plates are known 
by tlie name of whalebone^ and their general form 



and appearance, as they hang from the roof of 
the palate, are shown in Fig. 272, which repre- 
sents only six of these plates.* They are con- 
nected to the bone by means of a white liga- 
mentous substance, to which they are imme- 
diately attached, and from which they appear to 

grow : at their inner margins, 
the tibres, of which their tex- 
ture is throughout composed, 
cease to adhere together ; but, 
being loose and detached, 
form a kind of fringe, calcu- 
lated to intercept, as in a sieve, 
all solid or even gelatinous 
substances that may have been 
admitted into the cavity of the 
mouth, which is exceedingly 
capacious; for as the plates 
of whalebone grow only from 
the margins of the upper jaw, 
they leave a large space with- 
in, which though narrow an- 
teriorly is wider as it extends 
backwards, and is capable of 
holding a large quantity of water. Thus the 
whale is enabled to collect a whole shoal of mol- 

* la the Piked Whale the plates of whalehone are placed 
▼ery near together, not being a quarter of an inch asunder ; and 
there are above three hundred plates in the outer rows on each 
side of the mouth. 


lusca, and other small prey, by taking into its 
mouth the sea water which contains these ani- 
mals, and allowing it to drain off through the 
sides, after passing through the interstices of the 
net work formed by the filaments of the whale- 
bone. Some contrivance of this kind was even 
necessary to this animal, because the entrance 
into its oesophagus is too narrow to admit of the 
passage of any prey of considerable size ; and it 
is not furnished with teeth to reduce the food 
into smaller parts. The principal food of the 
Sal^ena MysticetuSy or great whalebone whale of 
the Arctic Seas, is the small Clio Borealis^ 
which swarms in immense numbers in those 
regions of the ocean ; and which has been al- 
ready delineated in Fig. 120.* 

These remarkable organs for filtration entirely 
supersede the use of ordinary teeth; and ac- 
cordingly no traces of teeth are to be discovered 
either in the upper or lower jaw. Yet a ten- 
dency to conform to the type of the mammalia 
is manifested in the early conformation of the 
whale; for rudiments of teeth exist in the in- 
terior of the lower jaw before birth, lodged in 
deep sockets, and forming a row <m each side. 
The developement of these imperfect teeth pro- 
ceeds no farther ; they even disappear at a very 
early period, and the groove which contained 

• Vol.i. p. 258. 


them closes over, and after a short time can no 
longer be seen. For the discovery of this 
curious fact we are indebted to Geofiroy St. 
Hilaire.* In conBexion with this subject, an 
analogous fact . which has been noticed in the 
parrot may here be mentioned. The young of 
the parrot, while still in the egg, presents a row 
of tubercles along the edge of the jaw, in ex* 
temal appearance exactly rraembling the rudi* 
meats of teeth, but without being implai^ted 
into regular sockets in the maxillary bones: 
they are formed, however, by a. j^ocess precisely 
similar to that of dentiti<m ; that is, by depo- 
ntion from a vascular pulp, connected with the 
jaw. These tubercles are afterwards consoli- 
dated into one piece in each jaw, forming by 
their union the beak of the parrot, in a manner 
perfectly analogous to that which leads to the 
construction of the compound tooth of the ele- 
phant, and which I shall presently describe. 
The original indentations are obliterated as the 
beak advances in growth; but they are per- 
manent in the bill of the duck, where the 
structure is v^ry similar to that above described 
in the embryo of the pamrt. 

* Cuvicr, Ossemens Fossiles, dme edition, tom. v. p. 360. 


^ 3. Mastication by means of Teeth. 

The teeth, being essential instruments for seizing 
and holding the food, and effecting that degree 
of mechanical division necessary to prepare it 
for the chemical action of the stomach, perform, 
of course, a very important part in the economy 
of most animals; and in none more so than in 
the Mammalia, the food of which generally re- 
quires considerable preparation previous to its 
digestion. There exist, accordingly, the most 
intimate relations between the kind of food 
upon which each animal of this class is intended 
by nature to subsist, and the form, structure, 
and position of the teeth; and these relations 
may, indeed, be also traced in the shape of the 
jaw, in the mode of its articulation with the 
head, in the proportional size and distribution of 
the muscles which move the jaw, in the form of 
the head itself, in the length of the neck, and its 
positi(m on the trunk, and indeed in the whole 
conformation of the skeleton. But since the 
nature of the appropriate food is at once indi- 
cated by the structure and arrangement of the 
teeth, it is evident thdt these latter organs, in 
particular, will afford to the naturalist most im- 
portant characters for establishing a systematic 
classification of animals, and more especially of 
quadrupeds, where the differences among the 


teeth are very considerable; and these differ- 
ences have, accordingly, been the object of much 
careful study. To the physiologist they present 
views of still higher interest, by exhibiting most 
striking evidences of the provident care with 
which every part of the organization of animals 
has been constructed in exact reference to their 
respective wants and destinations. 

The purposes answered by the teeth are prin- 
cipdlly those of seizing and detaining whatever 
is introduced into the mouth, of cutting it 
asunder, and dividing it into smaller pieces, of 
loosening its fibrous structure, and of breaking 
down and grinding its harder portions. Occa- 
sionally some particular teeth are much enlarged, 
in order to serve as weapons of attack or of 
defence ; for which purpose they extend beyond 
the mouth, and are then generally denominated 
tusks; this we see exemplified in the Elepluinty 
the Nanchalj the Wabmsy the HippopotamuSj 
the JBoaTy and the Bahiraussa. 

Four principal forms have been given to teeth, 
which accordingly may be distinguished into 
the conical, the sharp-edged, the fiat, and the 
tuberculated teeth ; though we occasionally find 
a few intermediate modifications of these forms. 
It is easy to infer the particular functions of 
each class of teeth, frotn the obvious^ mechanical 
actions to which, by their form, they are espe- 
cially adapted. The conical teeth, which are 


generally also sharp-pointed, are principally em- 
ployed in seirang, piercing, and holding objects: 
snch are the offices which they perform in the 
Crocodile, and othej Saurian reptiles, 'wiiere all 
the teeth are of this structure ; and such are also 
their uses in most of the Cetacea, where similar 
forms and arrangements of teeth prevail. All 
the Dolphin tribe, such as the Porpus, die 
Ch-anqna, and the Dolphin, are fumiAed vith a 
uniform row of conical teeth, set round both 
jaws, in number amounting frequently to two 
hundred. Fig. 273, which represents the jawB 
of the Porpus, shows the fonn of these umply 

prehensile teeth. The Cac/talot has a similar 
row of teeth, which are, however, confined to the 
lower jaw. All these animals subsist upon fish, 
and their teeth are therefore constructed very 
much on the model of those of fish ; while those 
Cetacea, on the other hand, which are her- 
bivorous, as the Manaius and the Dugong, or 
Indian Walrus, have teeth very differently 
formed. The tusks of animals must necessarily, 
as respects their shape, be classed among the 
conical teeth. 


The sharp^edged teeth perform the office of 
catting and dividiog the yielding textures pre- 
sented to them ; they act individually as wedges 
or cdiis^; but when co-operating with similar 
teeth in the opposite jaw, they have the power 
0f cutting like shears or scissors. The flat teeth, 
of which the surfaces are generally rough, are 
used in conjunction with those meeting them in 
the opposite jaw for grinding down the food by 
a lateral motion, in a manner analogous to the 
^[leration of mill-stones in a milL The tuber- 
calated teeth, . of which the surfaces present a 
number of rounded eminences, corresponding to 
depressions in the teeth opposed to them in the 
other jaw, act more by their direct pressure in 
breaking down hard substances, and pounding 
theni, as they would be in a mortar. 

The position of the teeth in the jaws has been 
another ground of distinction. In those Mam- 
malia which exhibit the most complete set of 
teeth, the foremost in the row have the sharp- 
edged OS chisel shape, constituting the blades of 
a cutting instrument ; and they are accordingly 
denominated incisors. The incisors of the upper 
jaw are always imjdanted in a bone, intermediate 
between the two upper jaw bones, and called 
the intermaxillary bones/ The conical teeth, 

* Those teeth of the lower jaw which correspond with the 
inciaors of the upper jaw, are also considered as incisors. In 
Man, and in the species of quadrumana that most nearly re- 


immediately following the incisors, are called 
cuspidate, or canine teeth, from their being par- 
ticularly conspicuous in dogs; as they are, in- 
deed, in all the purely camiyorous tribes. Iii the 
larger beasts of prey, as the lion and the tiger, 
they become most powerful weapons of destruc- 
tion : in the boar they are likewise of great 
size, and constitute the tusks of the animal. All 
the teeth that are placed farther back in the 
faw are designated by the general name of molar 
teeth, or grinders, but it is a class which includes 
several different forms of teeth. Those teeth 
which are situated next to the canine teeth, 
partake of the conical form, having pointed emi- 
nences ; these are called the false molar teeth, 
and also, from their having generally two points, 
or cusps, the bicuspidate teeth. The posterior 
molar teeth are differently shaped in carnivorous 
animals, for they are raised into sharp and often 
serrated ridges, having many of the properties 
of cutting teeth. In insectivorous and fru- 
givorous animals their surface- presents pro* 
minent tubercles, either pointed or rounded, for 
pounding the food; while in quadrupeds that 
feed on grass or grain they are flat and rough, 
for the purpose simply of grinding. 

The apparatus for giving motion to the jaws 

semble him, the sutures which divide the intermaxillary from the 
maxillary bones are obliterated before birth, and leave in the 
adult no trace of their former existence. 

b VkewiBe vafietl ^ceMiAng to* tbe pdvticular* 
moT^ifieBfs reqniied to act upen tb^ fi>od m Ike 
different tribes, The articulation of tl)e lower 
jaw with the temporal bone of the sHnU ap« 
proaclKs to a hiBge joint ; but conaideFable lati? 
tade m alloweii to its motions by the interposi* 
tion o^ 91 momaJIH^ cartilage between the two 
8iirfacesi«f artiiralaiMn, an contrivance admirably 
answenn^tfte ftMsided puppoM. Hence,, in ad^ 
ditioQ to the pmudpei mfivements of opening 
and shutting, which are mad^ in a vertical 
direction, the lower jaw has also soma degree of 
mebility i«^ a horizontal of liatera} din^eetion, And 
k likewise capaiUe of being moved backiiMardd' 
or forwards^ to a* certain ei(«^f|t, The mwi^eei* 
whkb eifect lAt^ closing of the ja'VF are prinei^ 
pally the temper^ m^ tha masseter musdes'; 
tfe former occupying' the hi^llow of the temples^ 
tke faittpF ctmnerl&cig the lower angle of tite^ jaM^ 
wilk Ab zygomatic arch'. The lateral motdenfi^ 
rf the jMf are efieetedb by mu^es placed inVer^ 
oaDy betKi»een'liie'8id<eish(;^ the jaw aft^d' the basi$ 
rfthe skuil, 

In tte c^cmibfmatiKiti of tftie «eetii» and j^^^, f|^ 
raniHlnbla contrast is i^^esentedi bet^wiaei) caav 
iHwvouS' and hn^ivorous^ animriB* Jb itte- for^ 
MP, of whicfa tAe 7%as Fig. 374, m^y be take^ 
as an example, the whole apparatus for masti^ 
cation is calculated for the destruction of life, 
and for tearing and dividing the fleshy fibpesr 

VOL. II, h 


The molar teeth are armed with pointed emi- 
nences, which correspond in the opposite jaws. 

so as exactly to lock, into one another, like 
whedwork, when the mouth is closed. All the 
muscles which close the jaw are of enormous 
size and strength, and they imprint the bones 
of the skull with deep hollows, in which we 
trace marks of the most powerful action. The 
temporal muscles occupy the whole of the sides 
of the skull (t, t) ; and by the continuance of 
their vigorous exertions, during the growth of 
the animal, alter so considerably the form of the 
bones, that the skulls of the young and the old 
animals are oflen with difficulty recognised as 
belonging to the same species.* The process of 
the lower jaw (seen between t and t), to which 
this temporal muscle. is attached, is large aad 

' Thia is remarkably the caae with the Bear, the skull of 
which exhibits in old animals a large vertical crest, not met with 
at u) early period of life. 


fKTHDiDent; and the arch of hone (z), from which 
the masseter arises, takes a wide span outwards, 
M as to gire great strength to the muscle. The 
cmidyle, or articulating surface of the jaw (c), is 
received into a deep cavity, cMistituting a strictly 
binge joint, and admitting simply the motions of 
opening and shutting. 

In herbivorous animals, on the contrary, as 
nay be se«i in the skull of the Antelope, Fig. 
275, the greatest force is bestowed, not so much 

00 the motions of opening and shutting, as on 
tlioBe which are necessary for grinding, and 
which act in a lateral direction. The temporal 
mnscles, occupying the space t, are compara- 
tifely small and feeble ; the condyles of the jaw 
are broad and rounded, and more loosely con- 
nected with the skull by ligaments ; the muscles 
in the interior of the jaw, which move it from 
aide to side, are very strong and thick ; and the 
bone itself ts extended downwards, so as to 


affonl ihem a broad basiis of attachmettt. Tk« 
surfaces of the molar teeth ok flattened and of 
great extent, and they ajre at the same time kept 
rough, like those of mitl-stones, their- office 
being in iact veiry aimilar to that performed by 
these implements for grinding. All these cir- 
cumstances of difierence are exemplified' in. the 
most marked manner, ia comparing together the 
skulls of the larger beasts of prey, a» the tiger* 
die wolf, or the bear, with those of the antelope, 
the horse, or the ox. 

The Rodentia, or gnawing quadrupeds, which 

I have already had occasion to notice, compose 

a well-marked &mily of Mammalia. These 

animals are formed for subsisting on dry and 

tough materials, from which but little nutriment 

can be extracted; such as the bark, and roots, 

and even the woody fibres of trees, aod the 

harder animal textures, which would ^pear to 

be most difficult of digestion. They are all 

animals of diminutive statuipe, whose teetlu are 

expressly fonaed for 

gnawing, nibbling, 

and wearing away by 

continued tkttriti<ui, 

the harder textures 

of organized bodies. 

The Rat, whose skuU 

is delineated in Fig. 276, belongs to this tribe. 

They are all iurnished withi two iuoisor teetb in 


each jaWy generally very long, and having the 
exact shape of a chisd ; and the molar teeth 
have isurfoces irregularly maiiced with raised 
sig-zag lines, rendering them very perfect in^ 
itriHnents of trituration. The zygomatic arch is 
exceedingly slender and feeble ; and the condyle 
is lengthened longitudinally to allow of the jaw 
beii^ £reely moved forwards and backwards, 
which k the motion for which the muscles are 
adapted, and by which the grinding operation is 
performed. The Beaver^ the Rat^ the Marmot^ 
a&d the Porcupine, present examples of this 
stnicture, among the omnivorous rodentia : and 
Ae Hare, the Rabbit, the 'Shrew, among those 
that are principally herbivorous. 

The Qnadrwmana, or M<mkey tribes, approach 
nearest to the human structure in the confer* 
mation of iJieir teedi, which appear formed for 
a mixed kind of food, but are especially 
adapted to the consumption of the more esculent 
fraits. The other orders of Mammalia exhibit 
intermediate gradations in the structure of their 
teeth to those above described, corresponding to 
greater varieties in the nature of their food. Thus 
the teeth and jaws of the Hyaena are formed 
more especially for breaking down bones, and 
in so doing exert prodigious force ; and those of 
the Sea Otter have rounded eminences, which 
peculiarly fit them for breaking shells. 

The teeth, though composed of the same 


chemical ingredients as the ordinary bonesi, 
differ from them by having a greater density 
and compactness of texture, whence they derive 
that extraordinary degree of hardness which 
they require for the performance of their pecuUar 
office. The substances of which they are com- 
posed are of three different kinds : the first, 
which is the basis of the rest, constituting the 
solid nucleus of the tooth, has been considered 
as the part most analogous in its nature to bone, 
but from its much greater density, and from its 
differing from bone in the mode of its formation, 
the name of ivory has been generally given to it. 
Its earthy ingredient consists almost entirely of 
phosphate of lime, the proportion of the car- 
bonate of that earth entering into its composition 
being very small ; and the animal portion is 
albumen, with a small quantity of gelatin. 

A layer of a still harder substance, termed the 
enamel^ usually covers the ivory, and, in teeth of 
the simplest structure, forms the whole of their 
outer surface : this is the case with the teeth of 
man and of carnivorous quadrupeds. These two 
substances, and the direction of their layers, are 
seen in Fig. 277, which is the section of a simple 
tooth. E is the outer case of enamel, o the 
osseous portion, and p the cavity where the 
vascular pulp which formed it was lodged. The 
enamel is composed almost wholly of phosphate 
of lime, containing no albumen, and scarcely a 


trace of gelatin ; it is the hardest of all animal 
sabstances, and is capable of atiiking Are with 

steel. It exhibits a fibrous structore, approach- 
ing to a crystalline arrangeoient, and the direc- 
tion )rf its fibres, as shown by the form of its 
fragments when broken, is every where perpen- 
dicular to the surface of the ivory to which it is 
applied. The ends of the fibres are thus alone 
exposed to the friction of the substances on 
which the teeth' are made to act ; and the effect 
of that friction in wearing the enamel is thus 
rendered the least possible. 

In the teeth of some quadrupeds, as of the 
Rhinoceros, the Hippopotamus, and most of the 
Rodentia, the enamel is intermixed with the 
ivory, and the two so disposed as to form jointly 
the surface for mastication. In the pn^iress of 
life, the layers of enamel, being the hardest, are 
le« worn down by liriction than those of the 
nory, and therefore fonn prominent ridges on 

1^ Tft£ VITAX. FUf<CTIOV«. 

4ie .0pin4tog ^ur&toa, {>WQei9iriiig it always m iliat 
fOMgh condition, wjbich best adapts it ior the 
bruising and comminuting of hard substances^ 

The incisors of the rodentia are guarded by a 
plate ^ enamel only on their anterior coDvex 
surfaces. «, that by the wearing down of the 
ivory behind this plate^ a wedge-like fonn, of 
wiiich the enamel constitutes the fine cutting 
edge, is ^soon given to the tooth) and is constantly 
retained ^s loiig as the tooth lasts (Fig^ 280). 
This mode of growth is admirably calculated to 
preserve these chisel teeth fit for use during the 
whole life-time of the ataimal^ aa object of greater 
consequence in this description of teeth th«m in 
others, which continue to grow only during a 
limited period. The same anrangement^ attended 
with simtlai' advantages, is adopted in the stme- 
tiire of the tusks of the Hippi^tamus. 

la teeth of a more complex stracture, a third 
substance is found, uniting the vertical plates^ 
ivory ^nd enamel, and performing the office «f 
an external cement. Thifi substanee has bo* 
ceived varioiis names, but it is most commonly 
}(.nDWJi by that of the Crusta petrosa: it resem^ 
bl^s ivory both in its composition and its extreioe 
hardpes^; but ifi geifierally more opaque and 
yellow than that substance. 

Other herbivorous quadrupeds, as the hovse, 
and aniiuals belongiug to the ruminant tribe, 
have also coi^plex t0etjbi.'conq^osed of these ttnrce 


; and th^ grinding surfaces present 
li^^ of enamel intermixed in a moce irregolar 
maHner with the hroiy and cmsta petrosa ; but 
sifll giring the advantage of a very rough surface 
ht tritnratian. Fig. 278 represents the grinding 
surface of the tooth of a horse, worn down by 
long jHBStteation. £ is the enamel, marked by 
tmmmsme Bmet^ showing the dnreetion of its 
filns, mul -enclosKQ^ the osseous porticm (o), 
wluiiis-ihwlad by interrupted lines. An outer 
coating of enamel (e) is also visible, and between 
that and di^e iimer coat, the substance called 
crusta peHmm (c), marked by waving lines, is 
seen. On ^dte aatside of all there is a plate of 
bone, which. juu» been left white. In ruminants, 
the plates of ^emamei form crescents, which are 
cmvex oQtwandly in the lower, and inwardly in 
the upper jaw ; thus providing for the crossing 
of the lidges of the two sur&ces, an arrange* 
meat aimilar to that which is practised in con-^ 
stnicting those of mill*stones. The teeth of the 
lower jMir fell within those of the upper jaw, so 
tbat a lateral motion is required in order to bring 
their sur&ces opposite to eaeh other alternately 
oa both sides. Fig. 279 shows the grinding sur« 
&ee of the tooth of a Sheep^ where the layers of 
bone are not apparent, there being only two layers 
of enamel (e), and one of crusta petrosa (c). 

These three component parts are seen to most 
adrraatage in a vertieal and longitudinal section 


of the grindiog tooth of the elephant, id which 
they are more completely and equally inter- 
mixed than in that of any other animal. Fig. 
281 presents a vertical section of the grinding 
tooth of the Asiatic Elephant, in the early stage 

of its growth, and highly polished, bo as to 
exhibit more perfectly its three component 
structures. The enamel, marked e, is formed of 
transverse fibres; the osseous, or innermost struc- 
ture is composed of longitudinal plates. The 
general covering of crusta petrosa, c, is leas 
r^ularly deposited, p is the cavity which had 
been occupied by the pulp. In this tooth, which 
is still in a growing state, the fangs are not yet 
added, but they are, at one part, banning to 
be formed. The same tooth in its usual state, 
as worn by mastication, gives us a natural and 


horizontal section of its interior structure, in 
which the plates of white enamel are seen 
fiimiing waved ridges. These constitute, in the 
Asiatic Elephant,, a series of narrow transverse 
hands (Fig. 283), and in the African Elephant, 
a series of lozenge-shaped lines (Fig. 282), hav- 
ing the ivory on their interior, and the yellow 
cnista petiosa on their outer sides ; which latter 
sabstance also composes the whole circumference 
of the section. 

\ 4. Formation and Developeineat of the Teeth. 

Few processes in animal developement are more 
remarkable than those which are employed to 
form the teeth; for they are by no means the 
same as those by which ordinary bone is con- 
structed ; and being commenced at a very early 
period, they afford a signal instance of Nature's 
provident anticipation of the futuilB necessities of 
the animal. The teeth, being the hardest parts 
of the body, require a peculiar system of opera- 
tions for giving them this extraordinary density, 
which no gradual consolidation could have im- 
parted. The formation of the teeth is in some 
respects analogous to that of shell ; inasmuch as 
all their parts, when once deposited, remain as 
permanent structures, hardly ever admitting of 
removal or of renewal by the vital powers. 


Unlike the b(meB» whicfa coataki withui dieir 
Bolid substance yessels of diffisreat kinds, by 
i??hieh they are nourished, modified, and occa- 
sionally remoyed, the ck>8eness of the texture of 
the teeth is such as to exclude all vessels what- 
soever. This circttmstajQce renders it necessary 
that they should originally be formed of the 
exact size and shape which they are ever after 
to possess: acowdingly the foundation of the 
teeth, in the young animal, are laid at a very 
early period of its evolution, and considerable pro- 
gress has been made in their growth even prior to 
birth, and long before they can come into use. 

A tooth of the simplest construction is formed 
from blood-vessels, which ramify through small 
masses of a gdatiaous appearance ; and each of 
these pulpy masses is itself enclosed in a delicate 
transparent vesicle, within which it grows till it 
has acquired the exact size and shape of the 
future tooth. Each vascular pulp is further 
jNTOtected by an investing membrane of greater 
strength, termed its capsule^ which is lodged in a 
small cavity between the two bony plates of the 
jaw. The vessels of the pulp begin at an early 
period to deposit the calcareous substance, which 
is to compose the ivory, at the most prominent 
points of that part of the vesicle, which corres- 
ponds in situation to the outer layer of the crown 
of the tooth. The thin scales of ivoty thus 
formed increase by farther depositions made on 


their suvfaees neat to the pulp, tUl the whole has^ 
formed the &P8t, or outer layer of ivory : in the 
mean time, the inner surfetce of the ca^nle, 
which is in immediate contact with this layer, 
secretes the substance that is to^ compose the 
eausd, and deposits it in; layers on the snrfiMe' 
of the ivory. This doable operation proceeds 
step by st^ fresh layers of ivory being depo* 
sited, aiMl building np the body of the toothy 
aad in the. same proportion encroaching upon> 
the cavity occupied by the pulp, which redoes^ 
befeve it, until it is shrunk into a small compass^ 
and fills, only the smaU cavity which remains in^ 
the centre of the tootb. The ivwy has by this- 
time received from the capsvde. a complete coaC^ 
iBg cS enamel, which consldtates the whole outer 
anrfiace of the crown ; bAct which no- move h 
deposited, and the function of the eapsule having 
ceased, it shrivels and disappears. But the* 
fiNmmtion of ivory still continuing at the part 
BHMt remote from the crown, Idie fangs are gra- 
dually fimned by a similar process from, ihe 
pulp; and a pressure beiag^ thereby dii^ected 
agianst the bone of the socket at the part where 
it is the tttinnest,. that portion of the jaw is" ab- 
serbed^ and the progress of the tooth is only 
resisted^ by the gum ; and the gum, in. its tum^ 
soon yiekling^ to the inereasing pressure, the 
tooth cuts its way to the surface. This process 
of successive deposition is beautifully illustrated 


by feeding a young animal at different times 
with madder; the teeth which are formed at 
that period exhibiting, in conseqaance, alternate 
layers of red and of white ivory.* 

The formation of the teeth of herbivorous 
quadrupeds, which have three kinds of substance, 
is conducted in a still more artificial and com- 
plicated manner. Thus in the elephant, the 
pulp which deposits the ivory is extended in the 
form of a number of paralM plates ; while the 
capsule which invests it, accompanies it in all 
its parts, sending down duplicatures of mem- 
brane in the intervals between the plates. Hence 
the ivory constructed by the pulp, and the 
enamel deposited over it, are variously inter- 
mixed ; but besides this, the crusta petrosa is 
deposited on the outside of the enamel. Cuvier 
asserts that this deposition is made by the same 
capsule which has formed the enamel, and which, 
previously to this change of function, has become 
more spongy and vascular than before. But 
his brother, M. Frederic Cuvier represents the 
deposit of crusta petrosa, as performed by a third 
membrane, wholly distinct from the two others, 
and exterior to them all, although it follows them 
in all their folds. In the horse and the ox, the 
projecting processes of the pulp, have more of a 
conical form, with undulating sides ; and hence 

* CuTier. Dictionnaire des Sciences Medicaks, t. viii. p. 3^0. 



the waved appearance presented by the enamel 
on making sections of the teeth of these animals. 

The tusks of the elephant are composed of 
iTory, and are formed precisely in the same 
maoner as the simple conical teeth already des- 
cribedy excepting that there is no outer capsule, 
and therefore no outer crust of enamel. The 
whole of the substance of the tusk is constructed 
by successive deposits of layers, having a ccmical 
shape, from the pulp which occupies the axis of 
the growing tusk ; just as happens in the forma- 
tion of a univalve shell which is not turbinated, 
as, for instance, the patella. Hence any foreign 
substance, a bullet, for example, which may 
happen to get within the cavity occupied by the 
pulp, becomes, in process of time, encrusted 
vith ivory, and remains embedded in the solid 
substance of the tusk. The pulp, as the growth 
of the tusk advances, retires in proportion as its 
place is occupied by the fresh deposits of ivory. 

The young animal requires teeth long before 
it has attained its fiill stature ; and these teeth 
must be formed of dimensions adapted to that of 
Uie jaw, while it is yet of small size. But as the' 
jaw enlarges, and the teeth it contains admit not 
of any corresponding incr^tse, it becomes neces- 
sary that they should be shed to make room for 
others of larger dimensions, formed in a more 
capacious mould. Provision is made for this 
necessary change at a very early period of the 


growth of the embryo. The rudimentei of the 
human teeth begin te fiMm feu« or fiye month* 
before birth : they are eontauied im the same 
sockets with the temporary teeth,, the oapauieft 
of both beifDg connected together. As Ihe jaw 
enlai^ed, the second set of teeth gradaally ac*- 
quire their fiiU dimensions^ and then^ by their 
outward pressure, occasion the absorption c£ the 
fhngs> of the temporally teathf, and, pushang thi^» 
put, occupy their places^* 

As the jaw bone, dusing its growth^ extends 
principttlly/' backwaidby the postenieir pwtm^ 
beingt later in fbnning, is comparatively of a 
larger size thaoi either the fore os the lateral 
parts* ; and it admits, therefore, of teeth of the 
fiill size, which consequently aise permanent 
The molar teeth, which are. last fionned, are, fi>r 
want of space, lauthes smaller than the otheiSi 
and are called the wisdom-teeth, because tbey 
do not usually make their appearanee; aheve 
the gum* till the pemea has aMained the ag|& of 
twenty. In the negro^ howeyer, whme the jew 
is of greater lengthy these teetb have sufficient 
room, to come into their places, and aM, in» gene'- 
ral, fiiUy a^ large as the other molafest 

The teelih. of camtvonous animals are, from 

* It is stated by Rousseau that the shedding of the first molar 
tooth both of the Guinea-pig, and the Capibara, and its re- 
placement by the permanent tocAh, take place tf few days betwP' 
birth. Anateraift Corapan^ du systAme deotaire, p. 164. 


the nature of their food, less liable to be worn, 
than those of animals living on grain, or on the 
harder kinds of vegetable substances ; so that 
the simple plating of enamel is sufficient to pre^ 
serve than, even during a long life. But in 
many herbivorous quadrupeds we find that in 
proportion as the front teeth are worn away in 
mastication, other teeth are formed, imd advance 
from the back of the jaw to replace them« This 
happens in a most remarkable manner in the 
d^ant, and is the cause of the curved form 
▼hich the roots assume ; for in proportion as the 
front teeth are worn away, those immediately 
hehind them are pushed forwards by the growth 
rfa new tooth at the back of the jaw ; and this 
process goes on continually, giving rise to a suc« 
cesfflon of teeth, each of which is larger than 
that which has i»eceded it, during the whole 
pmod that the animal lives. A similar sue- 
f^mon of teeth takes place in the wild boar^ and 
dso, though to a less extent, in the Sus (Ethio- 
fieu$* This mode of dentition appears to be 
peculiar to animals of great longevity, and 
which subsist on vegetable substances con- 
taining a large proportion of tough fibres, or 
^er materials of great hardness ; and requiring 
for their mastioation teeth so large as not to 
ftdmit of both the old and new tooth being 

• Home, PhU. Trans, for 1799, p. 237; and 1801, p. ai9- 


contained at the same time in the alveolar por- 
tion of the jaw. 

An expedient of a different kind has been 
resorted to in the Rodentia^ for the purpose of 
preserving the Icmg chisel-shaped incisors in a 
state fit for use. By the constant and sovere 
attrition to which they are exposed, they wear 
away very rapidly, and would soon be entirely, 
lost, and the animal would perish in conse- 
quence, were it not that nature has provided for 
their continued growth, by elongation from their 
rootsy. during the whole of life. This growth 
proceeds in the same manner, and is conducted 
on the same principles, as the original formation 
of the simple teeth already described : but, in 
order to effect this object, the roots of these 
teeth are of great size and length, and are 
deeply imbedded in the jaw, in a large bony 
canal provided for that purpose; and their cavity 
is always filled with the vasculai* pulp, from which 
the continued secretion and deposition of fi*esh 
layers, both of ivory and enamel, take place. 
The tusks of the Elephant and of the Hippo- 
potamw exhibit the same phenomenon of con- 
stant and uninterrupted growth. 

In the S/iarky and some other fishes, the same 
object is attained in a different mann^. Several 
rows of teeth are lodged in each jaw, but one 
only of these rows projects and is in use at the 
same time ; the rest lying fiat, but ready to rise 


hi order to replace those that have been broken 
or worn down. In some fishes the teeth ad- 
Tance in proportion as the jaw lengthens, and 
as the fore teeth are worn away : in other cases 
diey rise from the substance of the jaw, which 
prosents on its surface an assemblage of teeth in 
different stages of growth : so that in this class 
of animals the greatest variety occurs in the 
mode of the succession of the teeth. 

The teeth of the Crocodile^ which are shaip- 
pointed hollow cones, composed of ivory and 
enamel, are renewed by the new tooth (as is 
shown at a, in Fig. 284), being formed in the 

cavity of the one (b) which it 
29^4 is to replace, and not being 

inclosed in any separate cavity 

of the jaw bone (c). As this 

new tooth increases in size, it 

lllHAFIj presses against the base of 

the old one, and entering its 
cavity, acquires the same co- 
nical form ; so that when the 
latter is shed, it is already in 
its place, and fit for immediate use. This suc- 
cession of teeth takes place several times during 
the life of the animal, so that they are sharp and 
perfect at all ages. 

The fangs of serpents are furnished, Uke the 
stings of nettles, with a receptacle at their base 
for a poisonous liquor, which is squeezed out by 



the pressure of the tooth, at the moment it* 
inflicts the wound, and conducted along a canal, 
opening near the extremity of the tooth. Each 
fang is lodged in a strong bony socket, and is, 
by the intervention of a connecting bone, pressed 
forwards whenever the jaw is opened sufficiently 
wide ; and the fang is thus made to assume an 
erect position. As these sharp teeth are very 
liable to accidents, others are ready to supply 
their places when wanted: for which purpose 
there are commonly provided two or three half- 
grown fangs, which are connected only by soft 
parts with the jaw, and are successively moved 
forwards into the socket to replace those that 
were lost.* 

The tube through which the poison flows is 
formed by the folding in of the edges of a deep 
longitudinal groove, extending along the greater 
part of the tooth ; an interval being left between 
these edges, both at the base and extremity of 
the fang, by which means there remain apertures 
at both ends for the passage of the fluid poison. 
This structure was discovered by Mr. T. Smith 
in the Coluber naia^ or Cobra de Capello;1i and 
is shown in Fig. 385, which represents the fiiU 
grown tooth, where the slight furrow, indicating 
the junction of the two sides of the original 
groove, may be plainly seen; as also the two 

• Home, Lectures, &c. I. 333. 

t Philosophical Transactions^ 1818, p. 471. 



apertures (a and b) above mentioned. This 
mode of formation of the tube is farther illus- 
trated by Fig. 286, which shows a transverse 

section of the same tooth, exhibiting the cavity 
(p) which contains the pulp of the tooth, and 
vhich surrounds that of the central tube in the 
fonn of a crescent. Figures 287 and 288 are 
delineations of the same tooth in different stages 
of growth, the bases of which, respectively, are 
diown in Figures 289 and 290. Figores 291 
and 292 are magnified representations of sections 
of the fangs of another species of serpent, resem- 
bling the rattle-snake. Fig. 291 is a section of 
the young fang taken about the middle : in this 
stage of growth, the cavity which contains the 
pulp, almost entirely surroimds the poison tube, 
and the edges of the depression, which form the 
suture, are seen to be angular, and present so 
. large a surface to each other, that the suture is 


completely filled up, even in this early stage of 


growth. Fig. 292 is a section of a full-growa 
fenig of the same species of serpent, at the same 
part as the preceding; and here the cavity of 
the pulp is seen much contracted from the more 
advanced stage of growth. 

It is a remarkable circumstance, noticed by 
Mr. Smith, that a similar longitudinal furrow 
is perceptible on every one of the teeth of the 
same serpent ; and that this appearance is most 
marked on those which are nearest to the 
poisonous fangs : these furrows, however, in the 
teeth that are not venomous, are confined en- 
tirely to the surface, and do not influence the 
form of the internal cavity. No trace of these 
furrows is discernible in the teeth of those 
serpente which are not armed with venomoos 

Among the many instances in which teeth are 
converted to uses widely different from masti- 
cation, may be noticed that of the Sgualus pristis. 

or Saw-fidi, where the teeth are set hoiizontally 



on the two lateral edges of the upper jaw, which 
is prolonged in the form of a snout (seen in a, 
Fig. 293), constituting a most formidable weapon 
of offence, b is a more enlarged view of a 
portion of this instrument, seen from the under 

^ 5. Trituratum of Food in Internal Cavities. 

The mechanical apparatus, provided for tritu- 
rating the harder kinds of food, does not belong 
exclusively to the mouth, or entrance into the 
alimentary canal, for in many animals we find 
tUs office performed by interior organs. Among 
the inferior classes, we find examples of this 
conformation in the Crustacea, the MoUusca, 
and above all in Insects. Thus there is found 
in the stomach of the Lobster, a cartilaginous 
frame-work, in which are implanted hard cal- 
careous bodies, having the 
form, and performing the 
functions of teeth. They 
are delineated in Fig. 294, 
which presents a view of 
the interior of the sto* 
mach of that animal. The 
tooth A is situated in the 
middle of this frame, has a rounded conical 
shape, and is smaller than the others (b, c). 



which are placed one on each aide, and vhich 
lesemble in their form broad molar teeth. Wh«i 
these three teeth are brought tc^ether by the 
action of the surrounding muscles, they fit 
exactly into each other, and are capable of 
grinding and completely pulverising the shells 
of the moUusca introduced into the stomach. 
These teeth are the result of a secretirai of cal- 
careous matter from the inner coat of that organ, 
just as the outer shell of the animal is a pro- 
duction of the int^ument : and at each castii^ 
of the shell, these teeth, together with the whcie 
cuticular lining of the stomach to which they 
adhere, are thrown off, and afterwards renewed 
by a fresh growth of the same material. In the 
Craw-fish, the gastric teeth are of a difierent 
shape, and are more adapted to divide than to 
grind the food. 

Among the gasteropodous Mollusca, several 
species of Stdla have stomachs armed with 
calcareous plates, which act as cutting or grind- 
295 ing teeth. The Bulla aperta has 

three instruments of this descrip- 
tion, as may be seen in Fig. 295, 
which shows the interior of the 
stomach of that species. Similar 
organs are found in the Bulla 
lignaria. The Aplysia has a con- 
siderable number of these gastric teeth. An 
apparatus of a still more complicated kind is 



piOTided in most of the insects belonging to the 
order of Orthoptera ; but I shall not enter at 
present in their description, as it will be more 
conyenient to include them in the general ac- 
count of the alimentary canal of insects, which 
will be the subject of future consideration. 

The internal machinery for grinding is exem- 
plified on the largest scale in granivorous birds ; 
where it forms part of the stomach itself, and is 

termed a Gizzard. It is 
shown in Fig. 298, repre- 
senting the interior of the 
stomach of a Swan. Both 
the structure and the mode 
of operation of this organ 
bear a striking analogy 
to a mill for grinding 
com, for it consists of two 
powerful muscles (g)j of a 
hemispherical shape, with 
their flat sides applied to each other, and their 
edges united by a strong tendon, which leaves a 
THcant space of an oyal or quadrangular form 
brtween their two surfaces. These surfaces are 
covered by a thick and dense horny substance, 
which, when the gizzard is in action, performs 
an office similar to that of mill-stones. In most 
birds, there is likewise a sac, or receptacle, 
termed the Craw^ (represented laid open at c) in 
which the food is collected for the purpose of its 


merely by accident, or in consequence of the 
stupidity of the bird, which mistakes them for 
grain. But this opinion has been fiiUy and 
satisfactorily refiited both by Fordyce and by 
Hunter, whose observations concur in establishing 
the truth of the common opinion, that in all 
birds possessing ^zzards, the presence of these 
stones is essential to perfect digestion. A greater 
or less number of them is contained in every 
gizzard, when the bird has been able to meet 
with the requisite supply, and they are never 
swallowed but along with the food. Several 
hundred were found in the gizzard of a turkey ; 
and two thousand in that of a goose : so great 
an accumulation could never have been the 
result of mere accident. If the alleged mistake 
could ever occur, we should expect it to take 
place to the greatest extent in those birdd which 
are starving for want of food ; but this is far 
from being the case. It is found that even 
chickens, which have been hatched by artificial 
heat, and which could never have been instructed 
by the parent, are yet guided by a natural in- 
stinct in the choice of the proper materials for 
food, and for assisting its digestion : and if a 
mixture of a large quantity of sfones with a 
small proportion of grain be set before them, 
they will at once pick out the grain, and swallow 
along with it only the proper proportion of stones. 
The best proof of the utility of these substances 


may be derived from the experiments of Spal- 
lanzani himself, who ascertained that grain is 
IMA digested in the stomachs of birds, when it is 
protected firom the effects of trituration. 

Thus the gizzard may, as Hunter remarks, be 
regarded as a pair of jaws, whose teeth are taken 
in occasionally to assist in this internal mastica- 
tion. The lower part of the gizzard consists of 
a thin muscular bag, of which the office is to 
digest the food which has been thus triturated. 

Considerable differences are met with in the 
structure of the gizzards of various kinds of 
birds, corresponding to differences in the texture 
of their natural food. In the Turkei/^ the two 
muscles which compose the gizzard are of un- 
equal strength, that on the left side being consi- 
derably larger than that on the right ; so that 
while the principal effort is made by the former, 
a smaller force, is used by the latter to restore 
the parts to their situation. These muscles pro- 
duce, by their alternate action, two effects ; the 
one a constant trituration, by a rotatory motion ; 
the other a continued, but oblique, pressure of 
the contents of the cavity. As this cavity is of 
an oval form, and the muscle swells inwards, the 
opposite sidds never come into contact, and the 
interposed materials are triturated by their being 
intermixed with hard bodies. In the Goose and 
Swan^ on the contrary, the cavity is flattened, 
and its lateral edges are very thin. The surfaces 


applied to each other are mutually adapCed in 
their curvatures, a concave surface being every 
where applied to one which is convex : on the 
left side, the concavity is above; but on the 
right side, it is below. The homy covering is 
much stronger, and more rough than in the 
turkey, so that the food is ground by a sliding, 
instead of a rotatory motion of the parts opposed,, 
and they do not require the aid of any inter- 
vening hard substances of a large size. This 
motion bears a great resemblance to that of 
the grinding teeth of ruminating animals, in 
which the teeth of the under jaw slide upwards, 
within those of the upper, pressing the food he- 
tween them, and fitting it, by this peculiar kind 
of trituration, for being digested.* 

§ 6. Deglutition* 

The great object of the apparatus which is to 
prepare the food for digestion, is to reduce it 
into a soft pulpy state, so as to facilitate the 
chemical action of the stomach upon it : for 
this purpose, solid food must not only be sub- 
jected to mechanical trituration, but it must 
also be mixed with a certain proportion of fluid. 
Hence all animals that masticate their food are 

• Home, Phil. Trans, for 1810, p. 188. 


provided with organs which secrete a fluid, called 
the Saliva J and which pour this fluid ipto the 
mouth as near as possible to the grinding sur- 
faces of the t^eth. These oigans are glands, 
placed in such a situation as to be compressed 
by the action of the muscles which move the 
jaw, and to pour out the fluid they secrete in 
greatest quantity, just at the time when the food 
is undergoing mastication. Saliva contains a 
large quantity of water, together with some salts 
and a little animal matter. Its use is not only 
to soften the food, but also to lubricate the pas- 
sage through which it is to be conveyed into the 
stomach ; and the quantity secreted has always 
a relation to the nature of the food, the. degree 
of mastication it requires, and the mode in which 
it is swallowed. In animals which subsist on 
vegetable materials, requiring more complete 
maceration than those which feed on flesh, the 
salivary glands are of large size : they are paHi- 
colarly large in the Rodentia^ which feed on the 
hardest materials, requiring the most complete 
trituration ; and in these animals we find that the 
largest quantity of saliva is poured out opposite 
to the incisor teeth, which are those principally 
employed in this kind of mastication. In Birds 
and Reptiles^ which can hardly be said to mas- 
ticate their food, the salivary glands are compa- 
ratively of small size ; the exceptions to this rule 
occurring chiefly in those tribes which feed on 


vegetables, for in these the glands are more consi* 
derable.* In Fishes there is no structure of this 
kind provided, there being no mastication per- 
formed : and the same observation applies to the 
Cetacea. In the cephalopodous and gastero- 
podous Mollnscaf we find a salivary apparatus 
of considerable size : Insects, and the Annelidaylf 
also, generally present us with organs which 
appear to perform a similar office. 

The passage of the food along the throat i» 
facilitated by the mucous secretions, which are 
poured out from a multitude of glands inter- 
spersed over the whole surface of the membrane 
lining that passage. The Camel, which is formed 
for traversing dry and sandy deserts, where the 
atmosphere as well as the soil is parched, is spe- 
cially provided with a glandular cavity placed 
behind the palate, and which furnishes a fluid 
for the express purpose of moistening and lubri- 
cating the throat. 

In the structure of the (Esophagus, which m 
the name of the tube along which the food 
passes from the mouth to the stomach, we may 
trace a similar adaptation to the particular kind 
of food taken in by the animal. When it is 
swallowed entire, or but little changed, the 

* The large salivary gland in the woodpecker, is seen at s. 
Fig. 271, page 132. 

t The bunch of filaments, seen at s. Fig. 260 (p. 103) are the 
salivary organs of the leech. 


oesophagus is a very wide canal, admitting of 
great dilatation. This is the case with many 
carnivorous birds, especially those that feed on 
fishes, where its great capacity enables it to 
hold, tat a ccmsiderable time, the large fish which 
are swsdlowed entire, and which could not con- 
▼eniently be admitted into the stomach. Blu- 
menbach relates that a sea-gull, which he kept 
alive for many years, could swallow bones of 
three or four inches in length, so that only 
their lower ends reached the stomach, and were 
digested, while their upper ends projected into 
the oesophagus, and descended gradually in 
proportion as the former were dissolved. Ser- 
pents, which swallow animals larger than them- 
selves, have, of course, the oesophagus, as well 
as the throat, capable of great dilatation ; and 
the food occupies a long time in passing through 
it, before it reaches the digesting cavity. The 
tarde has also a capacious oesophagus, the inner 
coat of which is beset with numerous firm and 
sharp processes, having their points directed 
towards the stomach; these are evidently in- 
t^ided to prevent the return of the food into the 
Hiouth. Grazing quadrupeds, who, while they 
eat, carry their heads close to the ground, have 
a Icmg oesophagus, with thick muscular coats, 
capable of exerting considerable power in pro- 
pdling the food in the direction of the stomach, 
which is contrary to that of gravity. 



^ 7. Receptacles for retaining Food. 

Provision is often made for the retention of the 
undigested food in reservoirs, situated in different 
parts of the mouth, or the oesophagus, instead of 
its being immediately introduced into the sto- 
mach. These reservoirs are generally employed 
for laying in stores of provisions for future 
consumption. ' Many quadrupeds have cheek 
pouches for this purpose : this is the case with 
several species of Monkeys and Baboons; and 
also with the Mus cricetus^ or Hamster. The 
Mus bursarius^ or Canada rat, has enormous 
cheek pouches, which, when distended with food, 
even exceed the bulk of the head. Small cheek 
pouches exist in that singular animal, the Omi- 
thorhyncus. The Sciurus palmarum, or palm 
squirrel, is also provided with a pouch for laying 
in a store of provisions. A remarkable dilatation 
in the lower part of the mouth and throat, 
answering a similar purpose, takes place in the 
Pelican; a bird which displays great dexterity 
in tossing about the fish with which it has 
loaded this bag, till it is brought into the proper 
position for being swallowed. The Whale has 
also a receptacle of enormous size, extending 
from tlie mouth to a considerable distance under 
the trunk of the body. 



Analogous in design to these pouches are 
the dilatations of the oesophagus of birds, deno- 
minated crops. In most birds which feed on 
grain, the crop is a capacious globular sac, 
jdaced in front of the throat and resting on the 
furcidar bone. The crop of the Parrot is repre- 
sented at c. Fig. 299; where, also, s indicates 

the cardiac portion of the 
stomach, and g the giz- 
zard, of that bird. The 
inner coat of the crop is 
furnished with numerous 
glands, which pour out 
considerable quantities of 
fluid for macerating and 
softening the dry and 
hard texture of the grain, 
which, for that purpose, 
remains there for a considerable time. Many 
birds feed their young from the contents of the 
crop ; and, at those seasons, its glands are much 
enlarged, and very active in preparing their 
peculiar secretions : this is remarkably the case 
in the Pigeon (Fig. 300), which, instead of a 
single sac, is provided with two (seen at c, c. 
Fig. 300), one on each side of the oesophagus (o). 
The pouting pigeon has the faculty of filling 
these cavities with air, which produces that dis-^ 
tended appearance of the throat from which it 
derives its name. Birds of prey have, in general. 


very small crops, their food not requiring any 
previous softening ; but the Vulture^ which 
gorges large quantities of flesh at a single meal, 
has a crop of considerable size, forming, wh^i 
filled, a visible projection in front of the chest. 
Birds which feed on fish have no separate dila- 
tation for this purpose, probably because the 
great width of the oesophagus, and its having the 
power of retaining a lai^ mass of food, render 
the further dilatation of any particular part of 
the tube unnecessary. The lower portion of the 
oesophagus appears often, indeed, in this class of 
animals, to answer the purpose of a crop, and to 
efiect changes in the food which may properly 
be considered as a preliminary stage of the 
digestive process. 

Chapter VII. 

All the substances received as food into the 
stomach, whatever be their nature, must neces- 
sarily undergo many changes of chemical com- 
position before they can gain admission into the 
general mass of circulating fluids; but the extent 
of the change required for that pu]^>ose will, of 
course, be in proportion to the difference be- 


tween the qualities of the mitritiye materials in 
their original, and in their assimilated state. 
The conversion of vegetable into animal matter 
necessarily implies a considerable modification 
cf properties ; but even animal substances, how- 
ever similar may be their composition to the 
body which they are to nourish, must still pass 
through certain processes of decomposition, and 
sabeequent recombination, before they can be 
brought into the exact chemical state in which 
tfaey are adapted to the purposes of the living 

The preparatory changes we have lately been 
occupied in considering, consist chiefly in the 
reduction of the food to a soft consistence, which 
18 accomplished by destroying the cohesion of 
its parts, and mixing them uniformly with the 
fluid secretions of the mouth ; effects which may 
be considered as wholly of a mechanical nature. 
The first real changes in its chemical state are 
produced in the stomach, where it is converted 
into a substance termed Chyme ; and the process 
by which this first step in the assimilation of the 
food is produced, constitutes what is properly 
termed Digestum. 

Nothing has been discovered in the anato- 
mical structure of the stomach tending to throw 
any light on the means by which this remark- 
aUe chemical change is induced on the materials 
it contains. The stomach is in most animals 


a simple sac, composed of several membranes, 
enclosing thin layers of muscular fibres, abun- 
dantly supplied with blood-vessels and with 
nerves, and occasionally containing structures 
which appear to be glandular. The human sto- 
mach, which is delineated in Fig. 301, exhibits 

one of the simplest forms of this organ ; c being' 
the awdiac portion, or part where the oesophagus 
opens into it ; and p the pyloric portion, or that 
which is near its termination in the intestine. 
At the pylorus itself, the diameter of the pas- 
sage is much constricted, by a fold of the inner 
membrane, which is surrounded by a circular 
band of muscular fibres, performing the office of 
a sphincter, and completely closing the lower 
orifice of the stomach, during the digestion of 
its contents. 

The principal agent in digestion, as far as the 
ordinary chemical means are concerned in that 
operation, is a fluid secreted by the coats of the 


stomach, and termed the Gastric juice. This 
fluid has, in each animal, the remarkable pro- 
perty of dissolving, or at least reducing to a 
pulp, all the substances which constitute the na* 
tural food of that particular species of animal ; 
while it has comparatively but little solvent 
power over other kinds of food. Such is the 
conclusion which has been deduced from the 
extensive researches on this subject made by 
that inde&tigable experimentalist, Spallanzani, 
who found in numberless trials that the gastric 
juice taken from the stomach, and put into glass 
vessels, produced, if kept at the usual tempera- 
ture of the animal, changes to all appearance 
exactly similar to those which take place in 
natural digestion.^ In animals which feed on 
flesh, the gastric juice was found to dissolve only 
animal substances, and to exert no action on 
vegetable matter; while^ on the contrary^ that 
taken from herbivorous animals, acted on grass 
and other vegetable substances, without pro- 

• The accuracy of this conclusion has been lately contested 
l>y M. De Mont^re, whose report of the efiects of the gastric 
jnice of animals out of the body, does not accord with that of 
Spallanzani; but the difference of circumstances in which his 
experiments were made, is quite sufficient to account for the 
discrepancy in the results; and those of M. De Montegre, 
therefore, by no means invalidate the general facts stated in 
the text, which have been established by the experiments, not 
only of Spallanzani, but also of Reaumur, Stevens, Leuret, and 
Lassaigne. See Alison's Outlines of Physiology and Pathology, 
p 170. 


ducing any effect on fledh ; but in those animab^ 
which^ like man, are omnivorous, that is, par- 
take indiscriminately of both species of aliment, 
it appeared to be fitted equally for the solution 
of both. So accurate an adaptation of the che- 
mical powers of a solvent to the variety of sub- 
stances employed as food by different animals, 
displays, in the most striking manner, the vast 
resources of nature, and the refined chemistry 
she has put in action for the accomplishment 
of her different purposes. 

In the stomachs of many animals, as also in 
the human, it is impossible to distinguish with 
any accuracy the organization by which the 
secretion of the gastric juice is effected: but 
where the structure is more complex, there may 
be observed a number of glandular bodies inter- 
spersed in various parts of tilie internal coats of 
the stomach. These, which are termed the 
Gcistf^ic glands^ are distributed in various ways 
in different instances : they are generally found 
in greatest number, and often in clusters, about 
the cardiac orifice of the stomach ; and they are 
frequently intermixed with glands of another 
kind, which prepare a mucilaginous fluid, serving 
to protect the highly sensible coats of the sto- 
mach from injurious impressions. These latter 
are termed the mucous glands^ and they are often 
constructed so as to pour their contents into 
intermediate cavities, or small sacs, which are 


denominatei foUicles, where the fluid is collected 
bcfOTe it is dischaiged into the cavity of the sto- 
mach. The gastric glands of birds are larger 
and more coDspicuous than those of quadrupeds: 
tmt, independently of those which are situated 
in the stomach, there is likewise found, in 
ahnmt all birds, at the lower termination of the 
oesophagus, a large glandular organ, which has 
been termed the hulbulus glandulosas. In the 
Ostrich, this organ is of so great a size as to give 
it the appearance of a separate stomach. A 
view of the internal surface of the stomach of 
the African ostrich is given in Fig. 302 ; where 

c is the cardiac cavity, the coats of which are 
studded with numerous glands; o, o, are the 
two sides of the gizzard, fig. 303 shows one of 
the gastric glands of the African ostrich ; Fig. 
304, a gland from the stomach of the American 


ostrich ; and Fig. 306, a section of a gastric 
gland in the beaver, showing the branching of 
the ducts, which form three internal openings. 
In birds that live on vegetable food, the structure 
of the gastric glands is evidently differ^it from 
that of the corresponding glands in predaceous 
birds ; but as these anatomical details have not 
as yet tended to elucidate in any degree the pur- 
poses to which they are subservient in the pro- 
cess of digestion, I pass them over as being 
foreign to the object of our present inquiry.* 

It is essential to the perfect performance of 
digestion, that every part of the food received intd 
the stomach should be acted upon by the gastric 
juice ; for which purpose provision is made that 
each portion shall, in its turn, be placed in 
contact with the inner surface of that organ. 
This is the more necessary, as many facts ren- 
der it probable, as will be noticed more parti- 
cularly hereafter, that, besides the chemical 
action of the gastric juice, an influence, derived 
from the nerves, essentially contributes to the 
accomplishment of the chemical changes which 
the food undergoes in the stomach. For this 
reason it is that the coats of the stomach are 
provided with muscular fibres, passing, some 

* These structures have been examined with great care and 
minuteness by Sir Everard Home, who has given the results of 
his inquiries in a series of papers, read from time to time to the 
Royal Society, and published in their Transactions. 


ID a longitudinal, others in a transyerae, or 
circular direction ; while a third set have an 
oblique, or even spiral course.* When the 
greater number of these muscles act together, 
th^ exert a considerable pressure upon the 
ooDtents of the stomach ; a pressure which, no 
dcmbt, tends to assist the solvent action of the 
gastric juice. When different portions act in 
sueceaeion, they propel the food from one part 
to another, and thus promote the mixture of 
every portion with the gastric juice. We often 
find that the middle transverse bands contract 
BKNne strongly than the rest, and continue con- 
tracted for a considerable time. The object 
of this contraction, which divides the stomach 
into two cavities, appears to be to separate its 
OMitents into two portions, so that each may 
be subjected to different processes ; and, indeed, 
the differences in structure, which are often 
observable between these two portions of the 
stomach, would lead to the belief that their func- 
tions are in some respects different. 

During digestion the exit of the food from the 
stomach into the intestine is prevented by the 
pylorus being closed by the action of its sphinc- 
ter muscle. It is clear that the food is required 
to remain for some time in the stomach in order 
to be perfectly digested, and this closing of the 

* See Fig. 51, vol. i. p. 137, and its description, p. 138. 


pylonis appears to be one means employed for 
attaining this end ; a^d another is derived from 
the property which the gastric juice po s s es s e s of 
coagulating, or rendering solid, every animal or 
vegetable fluid susceptiUe of undergoing that 
change. This is the case with fluid albumen ; 
the white of an egg, for instance, which is 
nearly pure albumen, is very speedi]y coaga- 
lated when taken into the stomach ; the same 
change occurs in milk, which is immediately 
curdled by the juices that are there secreted, 
and these effects take place quite independently 
of any acid that may be present. The object 
of this change from fluid to solid appears to be 
to detain the food for some time in the stomach, 
and thus to allow of its being thoroughly acted 
upon by the digestive powers of that organ. 
Those fluids which pass quickly through the 
stomach, and thereby escape its chemical action, 
however much they may be in themselves 
nutritious, are very imperfectly digested, and 
consequently affi)rd very little nourishment. This 
is the case with oils, with jelly, and with all 
food that is much diluted.* Hunter ascer- 

* A diet consisting of too large a proportion of liquids^ 
although it may contain much nutritive matter, yet if it be 
incapable of being coagulated by the stomach, will not be 
sufficiently acted upon by that organ to be properly digested, 
and will not only afford comparatively little nourishment, but 
be very liable to produce disorder of the alimentary cand. Thus 



tained that this coagulating power belongs to 
the stomach of every animal, which he exa- 
mined for that purpose, from the most perfect 
down to reptiles^; and Sir E. Home has pro- 
secuted the enquiry with the same result, and 
ascertained that this property is possessed by the 
secretion from the gastric glands, which commu^ 
nicates it to the adjacent membranes*! 

The gastric juice has also the remarkable 
property of correcting putre&ction* This is par- 
ticBlarly exemplified in animals that feed on 
a«i<«: t. whL this i».p«rty i> of im- 
portance, as it enables them to derive wholesome 

tOQps win not prove no nutritive wben taken alone, as when 
they are united with a certain propOTtion of solid food, capable 
of being detained in the stomach, during a time sufficieutly long 
to allow of the whole undergoing the process of digestion. I was 
led to this conclusion, not only from theory, but from actual 
observation of what took place among the prisoners in the Mil- 
bank Peniteptiaryy in 1823* when on the occasion of the extensive 
prevalence of scorbutic dysentery in that prison, Dr. P. M. Latham 
and myself were appointed to attend the sick, and enquire into 
fSbe origin of the disease. Among the causes which concurred 
to pfodace this formidable malady, one of the most prominent 
qipeared to be an impoverished diet, consisting of a large 
proportion of soups, on which the prisoners had subsisted for the 
preceding eight months. A very full and perspicuous account 
of that disease has been drawn up, with great ability, by my 
fiieiid Dn P. M. Latham, and published under the title of ** An 
Accoont of the disease lately prevalent in the General Peniten- 
tiary.^ London, 1825, 

* Observations on the Animal Economy, p. 172. 

t Phil. Trans, for 1813, p. 96« 


Bourishment from materials which would other- 
wise taint the whole system with their poison, 
and soon prove destructive to life. 

It would appear that the first changes which 
constitute digestion take place principally at 
the cardiac end of the stomach, and that the 
mass of food is gradually transferred towards 
the pylorus, the process of digestion still con- 
tinuing as it advances. In the Rabbit it has 
been ascertained that food newly taken into 
the stomach is always kept distinct from that 
which was before contained in it, and which 
has begun to undergo a change : for this pur- 
pose the new food is introduced into the centre 
of the mass already in the stomach ; so that 
it may come in due time to be applied to the 
coats of that organ, and be in its turn digested, 
after the same change has been completed in 
the latter.* 

As the flesh of animals has to undei^o a less 
considerable change than vegetable materials, 
so we find the stomachs of all the purely carni- 
vorous tribes consisting only of a membranous 
bag, which is the simplest form assumed by 
this organ. But in other cases, as we have 
already seen, the stomach exhibits a division 
into two compartments, by means of a slight 

* See Dr. Philip's Experimental Enquiry into the Laws of 
the Vital Functions, 3d. edition, p. 122. 


coQtraction; a condition which, as Sir E. Home 
lias remarked, is sometimes found as a tem- 
porary state of the human stomach;* while, 
in <^er animals, it is the natural and per- 
manent confonnation. The Rodentia furnish 
maay examples of this dirision of the cavity 
into two distinct portions, which exhibit even 
differences in their structure : this is seen in the 
Domtouse, (Fig. 306) the Beaver, the Hare, the 
R<^bit, and the Cape Hyrax, (Fig. 307). The 
first, or cardiac portion, is often lined with 

cuticle, while the lower portion is not so lined ; 
as is seen very conspicuously in the stomachs of 
the SoUp^a. The stomach of the Horse, in 
particular, is furnished at the cardia, with a 

* T^e figure given of the human stomach, p. 182, shows it in 
ibt state of partial contraction here described. 


spiral fold of the inner, or cuticnlar membrane, 
which forms a complete valTC, offering no impe- 
diment to the entrance 
of food from the ceso- 
phagua, but obstruct' 
ing the return of any 
part of Uie contents oi 
the stomach into that 
passage.* This valTe 
is shown in Fig. 311, 
which represents an 
inner view of the car- 
diac portion of the sto- 
mach of the horse ; o 
being the termination of the cesophagus. 

The stomach of the Water Rat is composed 
of two distinct cavities, having a narrow passage 
of communication : the first cavity is lined with 
cuticle, and is evidently intended for the mace- 
ration of the food before it is submitted to the 
agents which are to effect its digestion ; a process 
which is completed in the second cavi^, pro- 
vided, for that purpose, with a glandular surface. 
In proportion as nature allows of greater lati- 
tude in diet, we find her providing greater c(»n- 
ptication in the digestive apparatus, and subdi- 
viding the stomach into a greater number of 

* The total inability of a horse to vomit is probably a conse- 
quence of the impedimeiit preseated by this valve. See Mem. 
du Mus^am d'Hiat. Nat. viii. 111. 


cavities, each having probably a separate office 
assigned to it, though concurring in one general 
eflect. A gradation in this respect may be 
traced through a long line of quadrupeds, such 
as the Hog^ ihePeccari, \he Porcupine, (Fig.308), 
and the Hippopotamus^ where we find the number 
of separate pouches for digestion amounting to 
four or five. Next to these we may rank the 
very irregular stomach of the Kanguroo, (Fig- 
300) composed of a multitude of cells, in which 
the food probably goes through several prepa- 
ratory processes : and still greater complication 
is exhibited by the stomachs of the Cetacea, as, 
for example, in that of the Porpus (Fig. 310). 
As the fishes upon which this animal feeds are 
swallowed whole, and have large sharp bones, 
which would injure any surface not defended by 
cuticle, receptacles are provided, in which they 
may be softened and dissolved, and even con- 
verted into nourishment, by themselves, and 
without interfering with the digestion of the soft 
parts. The narrow communications between 
these several stomachs of the cetacea are pro- 
bably intended to ensure the thorough solution 
of their contents, by preventing the exit of all 
such portions as have not perfectly undergone 
that process. 

Supernumerary cavities of this kind, be- 
longing to the stomach, are more especially 
provided in those animals which swallow food. 

VOL. II. o 


either in laiger quantity than is immediately 
wanted, or of a nature which requires much pre- 
paration previous to digestion. The latter is more 
particularly the case with the homed ruminant 
tribes that feed on the leaves or stalks of v^;e- 
tables, a kind of food, which, in proportion to its 
bulk, affords but little nutriment, and requires, 
therefore, a loi^ chemical process and a compli- 
cated digestive apparatus, iu order to extract from 
it the scanty nutritious matter it contains, and 
prepare it for beii^ applied to the uses of the 
system. This apparatus is usually considered 
as consisting of four stomachs ; and in order to 
convey a distinct idea of this kind of structure I 
have selected for representation, in Fig. 313, that 

of the Sheep, of which the four stomachs are 
marked by the numbers 1, 2, 3, 4, respectively, 
in the order in which they occur when traced 
from the oesophagus (c) to the intestine (p). 


The grasB which is- devoured in large quan- 
tities hy these animals, and which undergoes 
bat little mastication in the mouth, is hastily 
swallowed, and is receiyed into a capacious 
reservoir, marked 1 in the figure, called the 
pmmcA. This cavity is lined internally with a 
thick membrane, beset with numerous flattened 
papillae, and is often divided into pouches by 
traittvetse contractions. While the food remains 
in this bag, it continues in rather a dry state ; 
bat the moisture with which it is surrounded 
contrilmtes to soften it, and to prepare it for a 
second mastication ; which is effected in the 
following manner. Connected with the paunch 
is another, but much smaller sac (2), which is 
considered as the second stomach ; and, from its 
internal membrane being thrown into numerous 
irregular folds, forming the sides of polygonal 
cells, it has been called the honeycomb stomachy 
or reticule. Fig. 313 exhibits this reticulated 
appearance of the inner surface of this cavity. 
A singular connexion exists between this sto- 
madMmd the preceding ; for while the oesophagus 
app^rs to open naturally into the paunch, there 
is on each side of its termination, a muscular 
ridge which projects from the orifice of the latter, 
so that the two together form a channel leading 
into the sec<md stomach ; and thus the food can 
readily pass firom the oesophagus into either of 
these cavities, according as the orifice of the one 
or the other is open to receive it. 


It would appear, from the observations of Sir 
E. Home, that liquids drank by the animal pass 
at once into the second stomach, the entrance 
into the first being closed. The food contained 
in the paunch is transferred, by small portimis 
at a time, into this second, or honey-comb 
stomach, in which there is always a supply of 
water for moistening the portion of food intro- 
duced into it. It is in tliis latter stomach, th^, 
that the food is rolled into a baU, aiid thrown up, 
through the oesophagus, into the mouth, where it 
is again masticated at leisure, and while the ani- 
mal is reposing ; a process which is well known 
by the name of chewing the cud, or rumination. 

When the mass, after being thoroughly ground 
down by the teeth, is again swallowed, it passes 
along the oesophagus into the third stomach (3), 
the orifice of which is brought forwards by 
the m,»c«l« tands, forming the tw. ridge., 
already noticed, which are continued from the 
second stomach, and which, when they con- 
tract, effectually prevent any portion of the 
food from dropping into either of the preceding 
cavities. In the ox, this third stomach is de- 
scribed by Sir E. Home as ^having the form 
of a crescent, and as containing twenty-four 
septaj or broad folds of its inner membrane. 
These folds are placed parallel to one another, 
like the leaves of a book, excepting that they 
are of unequal breadths, and that a narrower 
fold is placed between each of the broader oneEU 


Fig. 314 represents this plicated structure in the 
interior of the third stomach of a bullock. 
Whatever food is introduced into this cavity, 
which is named, from its foliated structure, the 
wktt^-phes stanmcht must pass between these 
Mds, and describe three-fourths of a circle, 
before it can arrive at the orifice leading to the 
fourth stomach, which is so near that of the third, 
tkat the distance between them does not exceed 
three inches. There is, however, a more direct 
channel of communication between the ceso- 
phagus and the fourth stomach (4), along which 
milk taken by the calf, and which does not 
require to be either macerated or ruminated, is 
(XHQveyed directly from the oesophagus to this 
fourth stomach ; for at that period the folds of 
the many-plies stomach are not yet separated, 
and adhere closely together ; and in these ani- 
mals rumination does n6t take place, till they 
begm to eat solid food. It is in this fourth 
fltraiach, which is called the reedy that the proper 
digestion of the food is performed, and it is here 
that the coagnlation of the milk takes place ; on 
which account the c<^ts of this stdmach are 
employed in dairies, under the name of rennet ^ 
to obtain curd from milk. 

A regular gradation in the structure of rumi- 
nating stomachs may be traced in the different 
genera of this family of quadrupeds. In rumi- 
nants with horns, as the bullock and the sheep, 
there are two preparatory stomachs for retaining 


the food preyious ta rumination, a thud for 
receiying it after it has undergone this jproeess, 
and a fourth for effecting its digestion. Rumi- 
nants without horns, as the Camel, Dromedary, 
and Lama, have only one preparatory stomach 
before rumination, answering the purpose of the 
two first stomachs of the bullock; a second, 
which I shall presently notice, and which takes 
no share* in digestion, being employed merely as 
a reservoir of water ; a third, exceedingly small, 
and of which the office has not been ascertained ; 


and a fourth, which both receives and digests 
the food after ruinination. Those herbiywous 
animals which do not ruminate, as the horse and 
ass, have only <me stomach; but the upper 
portion of it is lined with cuticle, and appears 
to perform some preparatory office, which renders 
the food more easily digestible by the lower por- 
tion of the same cayity.* 

The remarkable provision above alluded to 
in the Camels an animal which nature has 
evidently intended as the inhabitant of the 
sterile and arid regions of the East, is that o{ 
reservoirs of water, which, when once £iUed, 
retain their contents for a very long time, and 
may minister not only to the wants of the animal 
that possesses it, but also to those of man. The 
second stomach of the Camel has a separate 

* Home, Phil. Trans. 8vo. 1806, p. 370. 


compartment, to which is attached a series of 
odlular appendages ; (exhibited on a small scale, 
in Fig. 315) : in these the water is retained by 
strong muscular bands, which close the orifices 
of the cells, while .the other portions of the 
stomach are perfonning their usual functions. 
By the relaxation of tiiese muscles, the water is 
gradually allowed to mix witii the c(mtents of 
the stomach, and thus the Camel is enabled to 
support long inarches across the desert without 
rec^ving any fresh supply. The Arabs, who 
traverse those extensive plains, accompanied by 
these use&l aninials, are, it is said, sometimes 
obliged, when faint, and in danger of perishing 
finun thirst, to kill* one oi their camels, for the 
sake of, the water contained in these reservoirs, 
which they always find to be pure and wholesome. 
It is stated by those who have travelled in Egypt, 
that camds, when accustomed to go journeys, 
during wiiich they are for a long time deprived 
of water, acquise the power of dUating the cells, 
80 .as to make them contain a more than ordinary 
qiumti^, as a supply for their journey.^ 

When the Elephant, while travelling in very 
hot weather, is tormented by insects, it has been 
observed to throw out from its proboscis, directiy 
upon the part 'on which the flies fix themselves, 
a quantity of water, with such force as to dislodge 

* Home, Lectures oq Comparative AnatOQiy^-voL i. p. 171. 


them. The quantity of water thrown out, is in 
proportion to the distance of the part attacked, 
and is commonly half a pint at a time : and this 
Mr. Pierard, who resided many years in India, 
has known the elephant repeat eight or ten times 
within the hour. The quantity of water at the 
animal's command for this purpose, observes Sir 
E. Home, cannot therefore be less than six 
quarts. This water is not only gected immedi*- 
ately after drinking, but six or eight hours after- 
wards. Upon receiving this information, Sir £. 
Home examined the structure of the stomach of 
that animal, and found in it a cavity, like that of 
the camel, perfectly well adapted to afford this 
occasional supply of water, which may, at other 
times, be employed in moistening dry food for 
the purposes of digestion.* 

In every series of animals belonging to other 
classes, a correspondence may be traced, as has 
been done in the Mammalia, between the nature 
of their food and the conformation of their diges- 
tive organs. The stomachs of birds, reptiles 
and fishes, are, with certain modifications, 
formed very much upon the models of those 
already described, according as the food con- 
sists of animal or of vegetable materials, or 
presents more or less resistance* from the co- 
hesion of its texture. As it would be impos- 

* Supplement to Sir £. Home's Lectures on Comparative 
Anatomy, vol. vi. p. 9. 


able in this place to enter into all the details 
necessary for fully illustrating this proposition^ 
I must content myself with indicating a few oif 
tbe most general results of the inquiry.^ 

As the food of birds yaries, in different spe* 
cies, from the softest animal matter to the 
hardest grain, so we observe every gradation in 
their stomachs, from the membranous sac of the 
carnivorous tribes, which is one extreme, to the 
trae gizzard of granivorous birds, which occu- 
pies the other extremity of the series. This 
gradation is established by the muscular fibres, 
which surround the former, acquiring, in dif- 
fident tribes, greater extent, and forming stronger 
mascles, adapted to the corresponding variations 
in the food, more especially as it partakes of the 
animal or vegetable character. 

In all the cold-blooded vertebrata, where di- 
gestion is not assisted by any internal heat, that 
operation proceeds more slowly, though in the 
end not less effectually, than in animals where 
the contents of the stomach are constantly main- 
tamed at a high temperature. They almost all 

* The comparative anatomy of the stomach has been investi- 
gated with great diligence by the late Sir £. Home, and the 
KsqIu recorded in the papers he communicated from time to 
time to the Royal Society, and which have been republished in 
Ub splendid work, entitled '^ Lectures on Comparative Anatomy," 
to which it will be seen that I have been largely indebted for the 
facts and observations relating to this subject, detailed in the 


rank as caroivorous animate, and have accoixl- 
ingly stomachs, which, however they may vary 
in their form, are alike simply membranous in 
their stnicture, and act by means of the solvent 
power of their 8ecreti<ms. Among reptiles, only 
a few exceptions occur to this rule. The 
common sea-turtle that is Immght to our tables^ 
is one of these ; for it is found to feed exclu* 
sively on vegetable diet, and chi^y on the sea* 
weed called zastira maritima^ and the structure 
of its stomach corresponds exactly to the gizzard 
of birds. Some tortoises, also, which eat grass, 
make an approach to the same structure. 

In fishes, indeed, although the membranous 
structure of the stomach invariably accompanies 
the habit of preying upon other fish, yet there is 
one species of animal food, namely, shell-fish, 
which require to be broken down by powerful 
means before it can be digested. In many fish, 
which consume food of this kind, its trituration 
is effected by ^ the mouth, which is, for this pur- 
pose, as I have already noticed in the wolf-fish, 
armed with strong grinding teeth. But in 
others, an apparatus similar to that of birds is 
employed ; the office of mastication bdng trans- 
ferred to the stomach. Thus the Mullet has a 
stomach endowed with a d^ree of muscular 
power, adapting it, like the gizzard of birds, to 
the double office of mastication and digestion ; 
and the stomach of the Gillaroo trouty a fish 


peculiar to Irdiand, exhibits, though iu a less 
degree^ the same structure. The common trout, 
abo, occasionally Uvea upon shell-fish, and 
swallows stones to assist in breaking the shells. 

AmoDg the invertebrated classes we occa*- 
aebaUy meet with instances of structures ex- 
ceedingly analogous to a gizzard, and probably 
perfonning the same functions. Such is the 
oigan found in the Sepia ; the earth-worm has 
both a crop and a gizzard; and insects ofier 
Bomerous instances, presently to be noticed, of 
great complexity in the structure of the stomach, 
which is often proyided, not only with a me- 
chanism analogous to a gizzard, but also with 
rows of gastric teeth. 

Chapter VIII. 

The formation of Chyle, or the fluid which is 
the immediate and exclusiye source of nutriment 
to the system, takes place in the intestinal tube, 
into which the chyme prepared by the stomach 
is leceived, and where farther chemical changes 
are effected in its composition. The mode in 
which the conversion of chyme into chyle is 
accomplished, and indeed the exact nature of the 
changes themselves, being, as yet, very imper- 


fectly known, it is consequently impossible to 
trace distinctly the correspondence which, in. all 
cases, undoubtedly exists between the objects^ 
to be answered, and the means employed for 
their attainment. No doubt can be entertained 
of the importance of the functions that are per- 
formed by structures so large and so complicated 
as are those composing the alimentary canal, 
and its various appendages. We plainly per* 
ceive that provision is made in the interior of 
that canal, for subjecting its contents to the 
action, first, of an extensive vascular and nervous 
surface ; and secondly, of various fluid secretions, 
derived from different sources, and exercising 
powerful chemical agencies on the digested 
aliment; that a muscular power is supplied, by 
means of the layers of circular and longitudinal 
fibres, contained between the outer and inner 
coats of the intestine,* for exerting a certain 
pressure on their contents, and for propelling 
them forwards by a succession of contractions, 
which constitutes what is termed their peristaltic 
motion ; and lastly, that contrivances are at the 
same time resorted to for retarding the progress 
of the aliment in its passage along the canal, so 
that it may receive the full action of these several 
agents, and yield the utmost quantity of nutri- 
ment it is capable of affording. 

• See vol. i. p. 137. 



The total length of the intestinal tube differs 
much in different animals, being in general, as 
ah'eady stated, smaller in the carnivorous tribes, 
than in those which feed on substances of diffi- 
cult digestion, or affording but little nourishment. 
In these latter animals, the intestine is always of 
great length, exceeding that of the body many 
times; hence it is obliged to be folded into a 
spiral or serpentine course, forming many con- 
volutions in the abdominal cavity. Sometimes, 
probably for greater convenience of package, 
instead of these numerous convolutions, a similar 
effect of increasing the surface of the inner 
membrane is obtained by raising it into a great 
number of folds, which project into the cavity. 
These folds are often of considerable breadth, 
amtribnting not only to the extension of the 
surface for secretion and absorption, but also to 
the detention of the materials, with a view to 
their more complete elaboration. Remarkable 

examples of this kind of struc- 
ture occur in iuost of the carti- 
laginous fishes, when the inner 
coat of the lai^e intestine is ex- 
panded into a broad fold, which, 
as is seen in Fig. 316, repre- 
senting this structure in the in- 
terior of the intestine of the 
shark, takes a spiral course ; and 
this is continued nearly the whole 


length of the canal, so that the internal surface 
is much augmented without any increase in Ihe 
length of the intestine.* 

When the nature of the assimilatory process 
is such as to require the complete detention of 
the food, for a certain time, in particular situa- 
tions, we find this object provided for by means 
of CiBcay or separate pouches, opening laterally 
from the cavity of the intestine, and having no 
other outlet. Stmctures of this description have 
already been noticed in the infusoria t^ and they 
are met with, indeed, in animals of every class, 
occurring in various parts of the alimentary tube, 
sometimes even as high as the pyloric portion of 
the stomach, and frequently at the commence- 
ment of the small intestine. Their most usual 
situation, howev«, is lower down, and especially 
at the part where the tube, after having remained 
narrow in the first half of its course, is dilated 
into a wider cavity, which is distinguished from 
the former by the appellation of the great intes- 
tine, and which is frequently more capacious 
than the stomach itself. It is exceedingly pro- 
bable that these two portions of the canal per- 
form difierent functions in reference to the 

* Stroctures of this diacription have a particular claim to 
attention from the light they throw on the nature of ieveral 
fossil remains, lately investigated with singular success by Dr. 

t Page 96, of this volume. 


aflfitinilatioii of the food: bat hitherto no clue 
has been discovered to guide ub through the 
intricacies of this difficult part of physiology ; 
and we can discern little more than the ex- 
istence, already mentioned, of a constant relation 
between the nature of the aliment and the 
structure of the intestines, which are longer, 
more tortuous, and more complicated, and are 
fomiahed with more extensive folds of the inner 
membrane, and with larger and more numerous 
caeca^ in animals that feed on vegetable sub- 
stances^ than in carnivorous animals of the same 

The class of insects supplies numberless 
exemplifications of the accurate adaptation of 
the structure of the organs of assimilation to the 
nature of the food which is to be converted into 
nufariment, and of the general principle that 
vegetable aliment requires longer processes, and 
a more complicated apparatus, for this purpose, 
than that which has been already animalized. 
In the herbivorous tribes, we find the oesophagus 
either extremely dilatable, so as to serve as a 
crop, or receptacle for containing the food pre- 
vious to its digestion, or having a distinct pouch 
appended to it for the same object : to this there 
generally succeeds a gizzard, or apparatus for 
trituration, furnished, not merely with a hard 
cuticle, as in birds, but also with numerous rows 
of teeth, of various forms, answering most effec^ 


tually the purpose of dividing, or grinding into 
the minutest fragments, all the harder parts of 
the food, and thus supplying any deficiency 
of power in the jaws for accomplishing the 
same object. Thence the aliment, properly 
prepared, passes into the cavity appropriated for 
its digestion, which constitutes the true sto- 
mach.* In the lower part of this organ a pecu- 
liar fluid secretion is often intermixed with it, 
which has been supposed to be analogous to the 
bile of the higher animals. It is prepared by 
the coats of slender tubes, termed hepatic 
vessels, which are often of great length, and 
sometimes branched or tufted, or beset, like the 
fibres of a feather, with lateral rows of filaments, 
and which float loosely in the general cavity of 
the body, attached only at their termination, 
where they open into the ieilimentary canal.-)* 

* It is often difficult to distinguish the portions of the canal, 
which correspond in their functions to the stomach , and to the 
first division of the intestines, or duodenum ; so that different 
naturalists, according to the views they take of the peculiar office 
of these parts, have applied to the same cavity the term otehy^ 
liferous stomachy or of duodenum. See the memoir of Leon 
Dufour, in the Annales des Sciences Naturelies, ii. 473. 

+ The first trace of a secreting structure, corresponding to 
hepatic vessels, is met with in the Asterias^ where the double row 
of minute lobes attached to the csecal stomachs of those animals, 
and discharging their fluid into these cavities, are considered by 
Cams, as performing a similar office. The fiocculent tissue 
which surrounds the intestine of the Holothuriay is probably 
also an hepatic apparatus. 


In some insects these tubes are of larger dia- 
meter than in otbers : and in many of the or- 
tlM^tera, as we shall presently see, they open 
iDto large receptacles, sometimes more capacious 
than the stomach itself, which have been sup^ 
posed to serve the purpose of reservoirs of the 
biliary secretion, pouring it into the stomach on 
those occasions only when it is ptuticularly 
wanted for the completion of the digestive 

The distinction into small and great intestine 
is more or less marked, in different insects, in 
proportion to the quantities of food consumed, 
and to its vegetable nature ; and in herbivorous 
tribes, more especially, the dilatations in the 
lower part of the canal are most conspicuous, 
as well as the duplicatures of the inner mem- 
brane, which constitute iipperfect valves for 
retarding the progress of the aliment. It is 
generally at the point where this dilatation of 
the canal commences, that a second set of 
hepatic vessels is inserted, having a structure 
essentially the same as those of the first set, but 
generally -more slender, and uniting into a small 
number of ducts before they terminate. The 
number and complication of both these sets of 
hepatic vessels, appear to have some relation to 

* A doubt is suggested, by Lk>n Dufour, whether the liquid 
fouod in those pouches is real bile, or merely alimeut in the pro* 
gnss of assimilation. Ann. Sc. Nat. ii. 478. 



the existence and deyelopement of the gizzard, 
and consequently also to the nature and bulk of 
the food. Vessels of this description are, indeed, 
constantly found in insects ; but it is only where 
a gizzard exists, that two sets of these secreting 
organs are provided ; and in some larvae, remark* 
able for their excessive voracity, even three 
orders ol hepatic vessels are met with.^ 

A muscular power has also been provided, not 
only for the strong actions exerted by the gizzard, 
but also for the necessary propulsion, in dif- 
ferent directions, of the contents both of the 
stomach and intestinal tubes. The muscular 
fibres of the latter are distinctly seen to consist 
of two sets, the one passing in a transverse or 
circular, and the other in a longitudinal direc- 
tion. Glandular structures, analogous to the 
mucous follicles of the higher animals, are also 
plainly distinguishable in the internal coat of the 
canal, more especially of herbivorous insects.f 
The whole tract of the alimentary canal is at- 
tached to the sides of the containing cavity by a 
fine membrane, or peritaneum, containing numer- 
ous air-vessels, or trache^B.X 

* See the Memoirs of Marcel des Serres, in the Annates du 
Museum, xx. 48. 

+ Lyonet. 

X It has been stated by Malpighi and by Swammerdaro, and 
the statement has been repeated by every succeeding ana- 
tomist, that almost all the insects belonging to the tribe of 



To engage in a minute description of the end- 
less variations in the structiue of the digestive 
organs, presented in the innumerable tribes 
which compose this class of animals, would 
be inoHnpatible with the limits of this treatise. 
I shall content myself, therefore^ with giving a 

few illustrations of their prin- 
cipal varieties, selected from 
those in which the leading 
characters of structure are 
most strongly marked. I shall, 
with this view, exhibit first one 
of the simplest forms of the 
alimentary organs as they oc- 
cur in the Mantis religiosa^ 
(Linn.) which is a purely car- 
nivorous insect, belonging to 
the order of Orthoptera. Fig. 
317 represents those of this 
insect, freed from their attach- 
ments, and separated from the 
body. The whole canal, as is 
seen, is perfectly straight : it 
commences by an oesophagus 
(o), of great length, which is succeeded by a 

^^m^ possessed the faculty of ruminating their food ; but this 
enorhas been refuted by Marcel des Serres, who has offered satis- 
factory evidence that in no insect is the food subjected to a true 
f^Qation, or second mastication, by the organs of the mouth. 
^ Annales du Museum, xx. 51 and 364. 



gizzard (g) ; at the lower extremity of this organ 
the upper hepatic vessels (b,b), eight in number, 
and of considerable diameter, are inserted : then 
follows a portion of the canal (d), which may be 
regarded either as a digesting stomach, or a 
chyliferous duodenum : farther downwards, the 
second set of hepatic vessels, (h h), which are 
very numerous, but as slender as hairs, are 
received : and after a small contraction (n) there 
is again a slight dilatation of the tube (c) before 
it terminates. 

The alimentary canal of the Cicindela campes- 
trisy (Lin.) which preys on other insects, is re- 
presented in Fig. 318 ; where we see that the 
lower part of the oesophagus (o), is dilated into 
a crop (p), succeeded by a small gizzard (g), 
which is provided for the purpose of bruising 
the elytra, and other hard parts of their victims : 
but, their mechanical division being once effected, 
we again find the true digesting stomach (s) 
simply membranous, and the intestine (i) very 
short, but dilated, before its termination, into a 
large colon (c). The hepatic vessels (h), of 
which, in this insect, there is only one set, ter- 
minate in the cavity of the intestine by four 
ducts, at the point where that canal commences. 

A more complicated structure is exhibited in 
the alimentary tube of the Melolontha vulgaris, 
or common cockchaffer, which is a vegetable 


feeder, devouring great quantities of leaves of 
plants, and consequently requiring a long and 
capacious canal for their assimilation ; as ia 
shown in Fig. 319, which represents them pre- 

pared in a similar manner to the former. In 
this herbivorous insect, the oesophagus (o) is, as 
aiight be expected, very short, and is soon dilated 
into a crop (p) ; this is followed by a very long, 
wide, and muscular stomach (s), ringed like an 


earth-wnn, and continued into a long and tor- 
^20 tuous intestine (i, i), which presents in 
its course several dilatations (c, c), 
and receives very elongated, convo- 
luted, and ramified hepatic vessels 
(h,h). Fig. 320 is a highly magnified 
view of a small portion of one of these 
vessels, showing its branched form. 
In the alimentary. canal (Fig. 321*) of the 
Acrida aptera (Stephens), 
which is a species of grass- 
hopper, feeding chiefly on the 
dewberry, we observe a long 
cesophagus (o), which is very 
jf X,'^* dilatable, enlaiging occasion- 

■" *" ceeded by a rounded or heart- 

shaped gizzard (o), of very 
complicated structure, and 
connected with two remark- 
ably large biliary pouches (u 
and b), which receive, at their 
anterior extremity, the upper 

set of hepatic vessels (vv). A 
deep furrow in the pouch (b), 
which, in the horizontal poei- 

• The figures reUling to this insect were engraTcd from the 
drawingfl of Mr. Newport, who was also kind enough to supply 
me with the description of tl»e parts they represent. Fig, 321 is 
twice the natural size. 


tion of the body, lies underneath the gizzard, 
divides it apparently into two sacs. The intes- 
tinal canal is pretty uniform in its diameter, re- 
ceives in its course a great number of hepatic 
vessels (h h)^ by separate openings, and after 
making one convolution, is slightly constricted 
at N, and is dilated into a colon (c), on the coats 
of which the longitudinal muscular bands are 
very distinctly seen. Fig, 322 is a magnified 
view of the gizzard laid open, to show its internal 
structure. It is furnished with six longitudinal 
rows of large teeth, and six intermediate double 
rows of smaller teeth ; the total number of teeth 
being 270. One of the rows of large teeth is 
seen, detached, and still more magnified, in Fig. 
323; it contains at the upper part, five small 
hooked teeth (f), succeeded below by four broad 
teeth (d), consisting of quadrangular plates, and 
twelve tricuspid teeth (t) ; that is, teeth having 
three cusps, or points at their edges. Fig. 324 
shows the profile of one of these teeth ; a, being 
the sharp point by which the anterior acute angle 
of the base terminates. Fig. 325 exhibits the 
base of the same tooth seen from below, e, e, e, 
being the three cusps, and m, the triangular 
hollow space for the insertion of the muscles 
which move them, and which compose part of 
the muscular apparatus of the gizzard. The 
smaller teeth, which are set in double lines 
between each of the larger rows, consist of twelve 


small triangular teeth in each row. All the 
teeth contained in this organ are of a brown 
colour and homy texture, resembling tortoise- 

The same insect, as we have seen, often 
exhibits, at different periods of its existence, 
the greatest contrast, not only in external form, 
but also in its habits, instincts, and modes of 
subsistence. The larva is generally remarkable 
for its voracity, requiring large supplies of food 
to furnish the materials for its rapid growth, and 
frequently consuming enormous quantities of 
fibrous vegetable aliment : the perfect insect, on 
the other hand, having attained its full dimen- 
sions, is sufficiently supported by small quantities 
of a more nutritious food, consisting either of 
animal juices, or of the fluids prepared by 
flowers, which are generally of a saccharine 
quality, and contain nourishment in a concen- 
trated form. It is evident that the same appa- 
ratus, which is necessary for ^he digestion of the 
bulky food taken in during the former period, 
would not be suited to the assimilation of that 
which is received during the latter ; and that in 
order to accommodate it to this altered condition 
of its function, considerable changes must be 
made in its structure. Hence^ it will be interest- 
ing to trace the gradual transitions in the ccmfor- 
mation of the alimentary canal, during the pro- 
gressive developement of the insect, and moire 


especially while it is undei^oing its different 

These changes are most conspicuous in th6 
Lepidoptera, where we may observe the suc- 
cessive contractions which take place in the im- 
mensdy voluminous Btoinach of the caterpillar, 
while passing into the state of chrysalis, and 
thence into that of the perfect insect, in which 
its form is so changed that it can hardly be 
recognised as the same organ. I have given re- 

presentations of these three different states of 
the entire alimentary canal of the Sphinx liguslri. 


or Privet Hawk-moth, in Figures 326, 327, and 
328* ; the first of which is that of the caterpillar ; 
the second, that of the chrysalis ; and the third, 
that of the moth. The whole canal and its ap- 
pendices, have been separated from their at- 
tachments, and spread out, so as to display all 
their parts; and they are delineated of the 
natural size, in each case, so as to show their 
comparative dimensions in these three states. 
In all the figures, a is the CBSophagus ; b, the 
stomach; c, the small intestine; d, the csecal 
portion of the canal ; and £, the colon, or large 
intestine. The hepatic vessels are shown at f ; 
and the gizzard, which is developed only in 
the moth, at g, Fig. 328. 

It will be seen that in the caterpillar, (Fig. 326), 
the stomach forms by far the most considerable 
portion of the alimentary tube, and that it bears 
some resemblance in its structure and capacity 
to the stomachs of the Annelida, already de- 
scribed.! This is followed by a large, but short, 
and perfectly straight intestine. These organs 
in the pupa (Fig. 327) have undergone con- 
siderable modifications, the whole canal, but 
more especially the stomach, beuig contracted 

* These figures also have been engraved from the drawings of 
Mr. Newport, which he was so obliging as to make for me, from 
preparations of his own, the result of very careful dissections. 

t See the figures and description of those of the Nais and 
the Leech, p. 102 and 103. 


both in length and width* : the shortening of 
the intestine not being in proportion to that 
of the whole body, obliges it to be folded upon 
Itself for a certain extent. In the moth, (Fig. 
328), the contraction of the stomach has pro- 
ceeded much farther ; and an additional cavity, 
which may be considered as a species of crop 
or gizzard (g), is developed : the small intestine 
takes a great many turns during its course, 
and a large pouch, or aecumy has been formed 
at the part where it joins the large intestine. 

The hepatic vessels are exceedingly nume- 
rous in the Crustacea, occupying a very large 
space in the general cavity ; and they compose 
by their union an organ of considerable size, 
which may be regarded as analogous in its 
functions to the Liver of the higher classes 
of animals. This organ acquires still greater 
size and importance in the MoUusca, where it 
frequently envelopes the stomach, pouring the 
bile into its cavity by numerous ducts.f As the 
structure and course of the intestinal canal 
varies greatly in different tribes of MoUusca, 
they do not admit of bemg comprised in any 

* Gams states that he found the stomach of a pupa, twelve 
days after it had assumed that state, scarcely half as long, and 
only one-sixth as wide as it had been in the caterpillar. 

t Transparent crystalline needles, the nature and uses of which 
are quite unknown, are frequently found in the biliary ducts 
of this class of animals. 


general descriptioD. The only examples I 
think it necessary to giTe, in this class, are those 
_ of the Patella, or Limpet, and 

3 of the PleurobroMckns. The in- 

testinal tube of the Patella is 
delineated in Fig. 329 ; where 
M is the mouth ; t, the tongue 
folded back ; o, the oesoph^us ; 
and s, the stomach, from which 
the tcntuous intestinal tube is 
seen to be continued. All the 
convolutions of this tube, as 
well as the stomach itself, are enclosed, or rather 
imbedded in the substance of the liver, which 
is the largest organ of the body. 

The Pleurobranckua Peronii (Cut.) is remark- 
able for the number and compli- 
cation of its organs of digestion. 
They are seen laid open in Fig. 
330 ; where c is the crop ; o, the 
gizzard ; p, a plicated stomach, re- 
sembling the third stomach of m- 
minant quadrupeds ; and d, a fourth 
cavity, being that in which diges- 
tion is completed. A canal of com- 
munication is seen at t, leading from 
the cr<^ to this last cavity : b is the 
point where the biUary duct enters. 

In the Cephalopoda, the structure of these 


organs is very complicated; for they are pro- 
Tided with a crop, a muscular gizzard, and a 
cscum, which has a spiral form. In these ani- 
mals we also discover the rudiment of another 
auxiliary organ, namely, the PancreaSy which 
secretes a fluid contributing to the assimilation 
of the food. This organ becomes more and more 
developed as we ascend in the scale of animals, 
assuming a glandular character, and secreting 
a watery fluid, which resembles the saliva, both 
in its sensible and chemical properties. It has 
been conjectured that many of the vessels, 
which are attached to the upper portion of the 
alimentary canal of insects, and have been 
termed hepatic, may, in fact, prepare a fluid 
having more of the qualities of the pancreatic 
than of the biliary secretion. 

The alimentary canal of fishes is in general 
characterised by being short ; and the con- 
tinuity of the stomach with the intestines is often 
8uch as to ofier no well marked line of distinc- 
tion between them. The caeca are generally 
large and numerous ; and a number of tubular 
organs, connected more especially with the 
pylorus, and called therefore the pyloric appen- 
dicesy are frequently met with, resembling a 
cluster of worms, and having some analogy, in 
situation at least, to the hepatic or pancreatic 
vessels of insects. Their appearance in the 


Salman is represented at p, in Fig. 33 1 . The pan- 

creas itself is only met with, in 
^^^ W^ ^^^ class of animals, in the order 
n of cartilaginous fishes, and more 
I especially in the Ray and the 
r Shark tribes. A distinct gall- 
bladder, or reservoir, is also met 
^1^ with in some kinds of fish, but is 
by no means general in that class. 
In the classes both of Fishes and of Reptiles, 
which are cold-blooded animals, the processes 
of digestion are conducted more slowly than in 
the more energetic systems of Birds and of 
Mammalia ; and the comparative length of the 
canal is, on the whole, greater in the former than 
in the latter : but the chief difierences in this 
respect depend *on the kind of food which is 
consumed, the canal being always shortest in 
those tribes that are most carnivorous.* As the 
Frog, in the different stages of its growth, lives 
upon totally different kinds of food, so we find 
that the structure of its alimentary canal, Uke 
that of the moth, undergoes a material change 
during these metamorphoses. The intestinal 
canal of the tadpole is of great length, and is 
collected into a large rounded mass, composed 
of a great number of coils, which may easily be 
distinguished, by the aid of a magnifying glass, 
through the transparent skin. During its gra- 

* See Home, Lectures, &c. I. 401. 


dnal (ransformation into a frog, this canal be- 
comes much reduced in its length ; so that when 
the animal has attained its perfect form, it 
makes but a single convolution in the abdominal 

A similar correspondence exists between the 
length of the canal, and the nature of the food 
in the class of Birds. At the termination of the 
small intestine there are usually found two cseca, 
which in the gallinaceous and the aquatic fowls, 
are of great length : those of the ostrich contain 
in their interior a spiral valve. Sir £. Home is 
of opinion that in these animals the functions 
of the pyloric portion of the stomach are per- 
foimed by the upper part of the intestine. 

In the intestines of the Mammalia contrivances 
are employed with the apparent intention of 
preventing their contents from passing along too 
hastily : these contrivances are most effectual in 
animals whose food is vegetable, and contains 
little nourishment, so that the whole of what the 
food is capable of yielding is extracted from 
them. Sir E. Home observes that the colon, or 
large intestine of animals which live upon the 
same species of food, is of greater length in 
proportion to the scantiness of the supply. Thus 
the length of the colon of the Elephant, which 
inhabits the fertile woods of Asia, is only 26i 
feet; while in the Dromedary, which dwells in 
the arid deserts of Arabia, it is 42. This con- 


trast is still more strongly marked in birds* 
The Cassowary of Java, which lives amidst a 
most luxuriant supply of food, has a colon of one 
foot in length, and two caeca, each of which is 
six inches long, and one quarter of an inch in 
diameter. The African ostrich, on the oth^- 
hand, which inhabits a country where the supply 
of food is very scanty, has the colon forty-five 
feet long; each of the cseca is two feet nine 
inches in length, and, at the widest part, three 
inches in diameter; in addition to which there 
are broad valves in the interior of both these 

On comparing the structure of the digestive 
organs of Man with those of other animals 
belonging to the class Mammalia, we find them 
holding a place in the series int^mediate be- 
tween those of the purely carnivorous, and ex* 
clusively herbivorous tribes ; and in some mea* 
sure uniting the characters of both. The powers 
of the human stomach do not, indeed, extend to 
the digestion either of the tough woody fibres of 
vegetables on the one hand, or . the compact 
texture of bones on the other ; but still they are 
competent to extract nourishment fi'om a wider 

* Lectures, &c. I. 470. In the account above given of the 
digestive organs I have purposely omitted all mention of the 
spleen ; because, although it is probably in some way related to 
digestion, the exact nature of its functions has not yet been 
determined with any certainty. 


range of alimentary substances, than the diges- 
tive organs of almost any other animal. This 
adaptation to a greater variety of food may also 
be inferred from the form and disposition of 
the teeth, which combine those of different kinds 
more completely than in most mammalia, ex- 
cepting, perhaps, the Quadrumana, in which, 
liowever, the teeth do not form, as in man, an 
uniDterrapted series in both jaws. In addition 
to these peculiarities we may also here observe 
that the sense of taste, in the human species* 
appears to be affected by a greater variety of 
objects than in the other races of animals. All 
these are concurring indications that nature, in 
thus rendering man omnivorous, intended to qua- 
lify him for maintaining life wherever he could 
procure the materials of subsistence, whatever 
might be their nature, whether animal or vege- 
table, or a mixture of both, and in whatever soil or 
climate they may be produced ; and for endow- 
ing him with the power of spreading his race, 
and extending his dominion over every acces- 
sible region of the globe. Thus, then, from the 
consideration of the peculiar structure of the 
vital, as well as the mechanical organs of his 
frame, may be derived additional proofs of their 
being constructed with reference to faculties of a 
higher and more extensive range than those of 
any, even the most favoured species of the brute 



Chapter IX. 


The Chyle, of which we have now traced the 
formatioD, is a fluid of umform coiisifltence« 
perfectly bland aud unirriitatiog in its properties, 
the elements of which have been brought into 
that precise state of chemical composition whieh 
renders them fit to be distributed to every 
part of the system for the purposes of nou^ 
rishment. In all the lower orders of animals 
it is transparent; bat the chyle of nammalia 
often contains a multitude of globules, which 
give it a white colour, like milk. Its chemical 
composition appears to be vary analogous to 
that of the blood into which it is afterwards con- 
verted: From some experiments made by my 
late much valued friend Dr. Marcet, it appears 
that the chyle of dogs, fed on animal food alone, 
is always milky, whereas in the same animals, 
when they are limite4 to a vegetable diet, it is 
nearly transparent and colourless.''^ 

The chyle is absorbed from the inner surface 
of the intestines by the Lacteals, which commence 

* Medico-Chirurgical Transactions ; vi. 630. 


by mf ttiifixrte orifices, in incalculable nuakbers, 
aad unite tfucceesiv^y into lajp^r and lai^er 
T6flBels, till they tontk titinktof ooi^Msiderable size. 
Hey pass between the folds 4>f a ^ery fine and 
delicate membrane, called th^ mesentery^ which 
ooDnects the intestines \o the spme, and which 
appears to be interposed in 4^rder to allow them 
that degree of freedom of motion, which is so 
neoessary to the prc^r p^ormance ftf their 
fimctions. la the* mesentery, the lacteals pass 
through sereral glandular bodies, termed the 
ma&aeric gland$j where it is probable that the 
efayle und^goes some modMcation, preparatory 
to its conversion iato blood. 

Hie mesenteric glands of the Whale contain 
large spherical cavities, into whidi the trun)c:s 
of the lacteals open, and where the chyle is 
probably blended with secretions proper to those 
cavities; but no similar structure can be de^^ 
tected in terrestrial mammalia. 

It is <Hily among the Vertebrata that lacteal 
veisels are met with. Those of Fudies are simple 
tabes, etthar .wholly without valves, or if there 
be any, th^ are in a rudimental i^ate, and 
Bot sufficiently extended to prevent the firee 
paasage 0t Xjx'&x fluid contents in a retrograde 
dmotiOB. The lacteals of the Turtle are larger 
and more distinct than those of fidies, Init their 
tbItcs are still imperfect, though they present 
some obstruction U> descendmg fluids. In Birds 


and in Mammalia these valres are perfectly 
efiectual» and are exceedingly numerous, giving 
to the lacteals, when distended with fluid, the 
appearance of strings of beads. The effect of 
these flood-gates, placed at such short intervals, 
is that every external pressure made upon the 
tube, assists in the propulsion of the fluid in the 
direction in which it is intended to move* Hence 
it is easy to imderstand how exercise must tend 
to promote the transmission of the chyle. The 
glands are more numerous and concentrated in 
the MammaUa, than in any other class. 

From the mesenteric glands the chyle is con- 
ducted, by the continuation of the lacteals, into 
a reservoir, which is termed the receptacle of the 
chyle: whence it ascends through the thoracic 
duct,'* which passes along the side of the spine, 
in a situation affording the best possible pro* 
tection from injury or compression, and opens into 
the great veins leading directly into the heart 

In invertebrated animals having a circulatory 
system of vessels, the absorption of the chyle is 
performed by veins instead of lacteal vessds. 

The sanguification of the chyle, or its convert* 
sion into blood, takes place during the course 
of the circulation, and is principally effected by 
the action of atmospheric air in certain organs^ 
hereafter to be described, where that action, or 

* This duct is occasionally douMe. 


aeratioH as it may be termed, m common with 
an analogous process in vegetables, takes place* 
In all vertebrated animals the blood has a red 
colour, and it is also red in most of the Anne- 
lida ; but in all other invertebrated animals, it 
IB either white or colourless.* We shall, for the 
pTesenty then, consider it as having undergone 
this change, and proceed to notice the means 
employed for its distribution and circulation 
throughout the systenu 

Chapter X< 


4 1 . Diffused Circtdatian. 

Animai^ life, implying mutual actions and re- 
actions l>etween the solids and fluids of the body, 
requires for its maintenance the perpetual trans- 
fer of nutritive juices from one part to another, 
c<nrresponding in its activity to the extent of the 
changes which are continuaUy taking place in 
the organized system* For this purpose we 

* Vanquelin bas observed tbat cbyle has often a red tinge in 


almost constantly fmd that a dscvi&tmy motkm 
of tbe nntrieiii fluids is established ; itad the 
ftmction which conducts and regnlates their 
moyements is emphatically denoBOLinafed the Vir* 
eulatian, Sereral objects of gfeat importance 
are answesed hf this fimctiom ; fcnv i^ the firet 
frface, it is through the circulation that e^ery 
organ is supplied with the nutritive particles 
necessary for its devdopement, its growth^ and 
the maintenance of its healthy condition; and 
that the glands, in particular, as well as the other 
secreting organs, are furnished with the materials 
they require for the elaboration of the products, 
which it is their peculiar office to prepare. A 
second essential object of the circulation, is to 
transmit the nutritiye juices to certain organs, 
where they are to be subjected to the salutary in- 
fluence of the oxygen of the atmosphere ; a pro- 
cess, which in all wann-btooded animals, com- 
bined with the rapid and extensiye distribution 
of the blood, difi'iBK^d and maintains throughout 
the system the high ti^mperatirye required by the 
greater etiergy of their .ftmetions. Hence it 
necessarily foHows that the {mtticcilar inode in 
which the circulation is conducted in each re- 
spectire tribe, mtist influence every other fone- 
tion of tbe economy, and must, therefore, consti- 
tute an essential element in determining the 
physiological condition of the animals We find, 
accordingly, that among the charactem oft 


whieh systematic zoologists have founded their 
great diyisions of the animal kingdom, the ut- 
most importance is attached to those derired 
fiom difierences of structure in the organs of cir- 

A comprehensive survey of the different classes 
of animals with reference to this function, enables 
us to discern the existence of a regular gradation 
of organs, increasing in complexity as we ascend 
from the lower to the higher orders ; and showing 
that here, as in other departments of the economy 
of nature, no change is made abruptly, but 
always by slow and successive steps. In the 
very lowest tribes of Zoophytes, the modes by 
which mitriticH^ is accomplished can scarcely be 
pereeived to differ from those adopted in the ve- 
getable kingdom, where, as we have already 
seeui the nutritive fluids, instead of being con- 
fijied in vessels, appear to permeate the cellular 
tissue; and tbas immediately supply the solids 
with the materials they require; for, in the 
simpler kinds of Polypi^ of Infusoria, of Medusee> 
and of Entozoa, the nourishment which has been 
prepared by the digestive cavities is apparently 
imbibed by the solids, after having transuded 
through the sides of these organs, and without its 
being previously collected into other, and more 
general cavities. This mode of nutrition, suited 
only to the torpid and half vegetative nature of 
t/oaphyteBf has been denominated nomriskment by 


imbihition^ in contradistinction to that by drtur 
latian ; a tenn, which, as we have seen, implies, 
not merely a system of canals, such as those ex- 
isting in Medusffi, where there is no evidence of 
the fluids really circulating, but an arrangem^t 
of ramified vessels, composed of membranous 
coats, through which the nutrient fluid moves in 
a continued circuit. 

The distinction which has thus been drawn, 
however, is one on which we should be careful 
not to place undue reliance, for it is founded, 
perhaps, more on our imperfect means of investi- 
gation, than on any real difierences in the proce- 
dures of nature relative to this function. When 
the juices, either of plants or of animals are trans- 
parent, their motions are imperceptible to the eye, 
and can be judged of only by other kinds of evi- 
dence ; but when they contain globules, difiering 
in their density from that of the fluid, and there- 
fore capable of reflecting light, as is the case 
with the sap of the Chara and CauHnia^ we have 
ocular proof of the existence of currents, which, 
as long as the plant is living and in health, pur- 
sue a constant course, revolving in a regular and 
defined circuit ; and all plants which have milky 
juices exhibit this phenomenon. Although the 
extent of each of these vegetable currents is very 
limited, compared with the entire plant, it still 
presents an example of the tendency which the 
nutrient fluids of organized structures have to 


move in a circuit, even when not confined within 
vessels or narrow channels ; for this movement of 
roiatiom^ or cyehsis^ as it has been termed,* what- 
ever may be its cause, appears always to have a 
definite direction* The current returns into 
itself, and contmues without intermission, in a 
manner much resembling the rotatory movements 
occasionally produced in fluids by electro-mag- 

Movements, very similar in their appearance 
and character to thos^ of vegetable cyclosis, 
have been recently discovered in a great 
number of polypiferous Zoophytes, by Mr. 
Lister, who has communicated his observa- 
tions in a paper which was lately read to the 
Royal Society, and of which the following are 
the principal results. In a specimen of the 
jTHhularia indivisa^ when magnified one hundred 
times, a current of patrticles was seen within the 
tabular stem of the polype, strikingly resem- 
bling, in the steadiness and continuity of its 
stream, the vegetable circulation in the Chara. 
Its general course was parallel to the slightly 
spiral lines of irregular spots on the surface of 
the tube, ascending on the one side, and de- 

* See pages 49 and 50 of this volume. 

t So great is this resemblance, that it has led several physiolo- 
gists to ascribe these movements to the agency of electricity ; but 
there does not, as yet, appear to be any substantial foundation for 
Ihis hypothesis. 


scending on the other; >each of the opposite 
currents occupying one-half of the circumfer«- 
ence of the cylindric cavity. At the knots, or 
contracted parts of the tube, slight eddies were 
noticed in the currents ; and at each end of the 
tube the particles were seen to turn round, and 
pass over to the other side. In rarious species 
of Serttdaria the stream does not flow in the 
same constant direction; but, after a time, its 
Telocity is retarded, and it then either stops, or 
exhibits irregular eddies, previous to its return in 
an opposite course; and so on alternately, like 
the ebb and flow of the tide. If the currents be 
designedly obstructed in any part of the stem, 
those in the branches go on without interruption, 
and independently of the rest. The most re- 
markable circumstance attending these streams 
of fluid is that they appear to traverse the cavity 
of the stomach itself, flowing from the axis of 
the stem into that organ, and returning into the 
stem with^ any vfeible cause determining, these 
movements. Similar phenomena were observed 
by Mr. Lister in CampantdariiB and PhtmidaH^e. 
In some of the minuter species of Crustacea 
the fluids have been seen, by the aid of the 
microscope, moving within the cavities of the 
body, as if by a spontaneous impulse, without 
the aid of a propelling organ, and apparently 
without being confined in membranous channels, 


tubes of any sort This kind of difibied cir* 
cidation is also seen in the embryos of Tarions 
animals, at the earliest periods of their develope- 
Mcaot, and bdbre any tessels are formed. 

§ 2. Vascular Cirenlatian. 

Thjc next step in the gradation of structures con* 
asts in the presence of vessels, within which the 
fluids are confined, and by which their course 
and their velocity are regulated ; and in general 
these v«sels fbr^ a co^ete circuit. Thefirst 
rudiments of a vascular organization are those 
observed and described by Tiedemann^ in the 
Asterup^ which are situated higher in the animal 
scale than Medusee; but whether any actual 
dicnlatioii takes place in the channels consti^ 
tnted by these vessels, which communicate both 
with the cavity of the intestine, and with the 
re spirat ory organs, is not yet determined with 
any certainty. The Hoiotkuri€B, which also 
bdoDg to the order of Echinodermata, are fur* 
flisked with a e<mipiex apparatus of vessels, of 
which the exact fonctions are still unknown. 
In tbo0e spedbs of Entozoa which exhibit a 
vascular structure, the canals appear rather to 
be ramifications of the intestinal tube, than 
proper vessels, for no distinct circtdation can be 


traced in them : an oi^anization of this kind has 
already been noticed in TisnuB^ 

It was, till very lately, the prevailing opinion 
among naturalists that all true insects are nou* 
rished by imbibition, and that there exists in 
their system no real vascular circulation of 
juices. In all the animals belonging to this 
class, and in every stage of their developement, 
there is found a tubular organ, called the dorsal 
vesself extending the whole length of the back, 
and nearly of uniform diameter, except where it 
tapers at the two ends. It contains a fluid, 
which appears to be undulated backwards and 
forwards, by means of contractions and dilata- 
tions, occurring in succession in different parts 
of the tube; and it is also connected with 
transverse ligamentary bands, apparently con- 
taining muscular fibres, capable by their action 
of producing, or at least of influencing these pul* 
satory movements. An enlarged representation 
of the dorsal vessel of the Mehhntha tmlgarisj 
or common cockchaffer, isolated firom its attach'^ 
ments, is given in Fig. 333, showing the series 
of dilatations (v, v, v) which it usually presents 
in its course ; and in Fig. 334 the same vessel is 
exhibited in connexion with the ligamentary and 

* Page 83, in this yolume ; Fig. 247. The family of Pb* 
wxnat present exceptions to this general rule : for many species 
possess a system of circulating vessels. See Dug^s, Annates 
des Sciences Naturelles; xv, 161. 


muscular apparatus which surroundB it, seen 
from the lower side. In the last of these figures, 

A is tlte tapering prolongation of the tube* pro- 
ceeding towards the head of the insect ; v, one of 
tbe dihited portions, or ventricles, as they have 
been called, of the dorsal part of the tube ; f, one 
(tf the small tendinous folds, to which the Uga- 
mentary bands are attached; and l is one of 
these bands, having a triangular, or, if considered 
as continuous with that on the other side of the 
Te8sel> a rhomboidal shape, and attached at b. 


to the superior flegments of the abdomen. At i 
is seen a layer of the aame fibres, whieh are 
partly ligamentous and partly muscidar, passing 
underneath the dorsal vessel, and forming, in 
conjunction with the layer that passes above it, 
a sheath, which embraces and fixes that vessel 
in its place: these inferior layars hare been 
removed fr<»n the other parts of the vessel, to 
allow the upper layers to be seen, as is the case 
at L. Fig. 335 gives a side view of the anterior 
extremity of the same vessel, showing the curve 
(a) which it describes as it bends downwards ia 
its course towards the head. 

The function performed by the dorsal vefusel, 
which, judging from the universal presence of 
this organ in insects, must be one of great im- 
portance in their economy, was long a profound 
mystery. Its analogy in structure and positioD 
to the dorsal vessels of the Arachnida and the 
Annelida, where it evidently conmiunicates with 
channels of circulation, and exhibits movements 
of pulsation resembling those of insects, was 
a strong argument in favour of the opinion that 
it is the prime mover of a similar kind of cirea^ 
lation ; but thai, again, this hypothesis w^ 
peared to be overturned by the fact that no 
vessels of any kind could be seen extaidnig 
from it in any direction ; nor could any channel 
f(Nr the transmission of a circulating fluid be 
detected in any part of the body. Those orgisfis, 


which, in animala aj^paxiently of an inferiw rafik^ 
ane most vaacular, such a3 the stomach, the 
iatestinal tube, the eye, and ^ther apparatus 
of the senses, seemed to be constnieted, and 
to be nourished^ by means totally different from 
dioee adopted in the former animals. Althoi^ 
extmndy minute rami&eations of air tubes aie 
every where visible in the interior of insects, 
yet, neither Cuvier, nor any other anatomist, 
could succeed, by the closest scrutiny, in de- 
tecting the least trace of blood vessels ; and the 
presumption, therefore, was, that none existed. 

But it still remained a question, if the dorsal 
Tesad be not subservient to circulation, what 
is its real function? Marcel des Serres, who 
bestowed g^eat pains in investigating this sub- 
ject, came to the conclusion that its use is to 
secrete the fatty matter, which is g^ierally 
iband in great abundance in the abdominal 
cavity, and which is accumulated particularly 
around the dorsal vessel.* A more attentive 
examination of the structure of the vessel itself 
brought to light a valvular apparatus, of which 
the only conceivable purpose is that of deter* 
mining the motion of the contained fluid in one 
constant course; a purpose necessarily incom- 
patible with ite supposed altemaie undulation 

* See his various papers in the M^moires du Museum d' Hist. 
Nat; iom iv. and v. 


in opposite directions^ from one end of the 
tube to the other. These valves are exhibited in 
Fig. 336, in a still more magnified view of a 
longitudinal section of the dorsal vessel, showing 
the semicircular folds (s, s) of its inner mem- 
brane, which perform the function of valves by 
closing the passage against any retrograde mo- 
tion of the fluid. This discovery of valves in 
the dorsal vessel, again made the balance of 
probability incline towards the opinion that it 
is the agent of some kind of circulation. 

All doubt as to the reality of a circulation in 
insects is now dispelled by the brilliant dis- 
coveries of Professor Carus, who, in the year 
1824, first observed this phenomenon in the 
larva of the Agrion puella. In the transparent 
parts of this insect, as well as of many others, 
numerous streams of fluid, rendered manifest 
by the motions of the globules they contain, 
are seen meandering in the spaces which inter- 
vene between the layers of the integument, 
but without appearing to be confined within 
any regular vessels. The streams on the sides 
of the body all pass in a direction backwards 
from the head, till they reach the neighbourhood 
of the posterior end of the dorsal vessel, towards 
which they all converge ; they are then seen to 
enter that vessel, and to be propelled by its pul- 
sations towards its anterior extremity, where they 
again issue firom it, and are subsequently divided 


into the scattered streams, which descend along 
the sides of the body, and which, after haying 
thus completed their circuit, retm*n into the pul- 
satiag dorsal vessel. 

This mixed kind of circulation, partly diffused 
and partly vascular, is beautifully seen in the 
larva of the Ephemera marginata^* where, be- 
sides the main current, which, after being dis- 
charged from the anterior extremity of the dorsal 
vessel, descends in a wide spreading stream 
on each side and beneath that vessel, another 
portion of the blood is conveyed by two lateral 
trunks, which pass down each side of the body, 
in a serpentine course, and convey it into the 
lower extremity of the dorsal vessel, with which 
they are continuous. These are decidedly ves- 
sels, and not portion^ of the great abdominal 
cavity, for their boundaries are clearly defined ; 
yet they allow the blood contained in them 
to escape into that cavity, and mix with the 
portion previously diffused. All these wandering 
streams sooner or later find their way into the 
dorsal vessel, being absorbed by it at various 
points of its course, where its membranous coat 
is reflected inwards to form the valves. In the 

* This insect is figured an<) described in Dr. Goring and 
Mr. Pritchard's " Microscopic Illustrations," and its circulation 
is very fully detailed, and illustrated by an engraving on a large 
scale, by Mr. Bowerbank, in the Entomological Magazine, i, 239 ; 
plate ii. 




legs, the tail, and the antenase, the circulatioD is 
carried on by means of vessels, which are con- 
tinuous with the lateral vessels of the body, 
branching off from them in the form of loops, 
ascending on one side, and then turning back to 
form the descending vessel, so that the currents 
in each move in contrary directions. Fig. 337 

represents the appearance of these parallel ves- 
sels in one of the antennae of the Semblis viridis, 
magnified thirty times its natural size. The 
whole system of circulating vessels in that in- 
sect, of which the former is only a detached 
part, is shown in Fig. 3.?8, where the course 
of the blood is indicated by arrows ; a, repre- 
senting the currents in the antennae ; w, those in 
the rudimental wings ; and T, those in the tail ; 
in all which parts the vessels form loops, derived 


from the main vessels of the trunk. In some 
larv8B the rascular loops, conveying these colla- 
teral streams, pass only for a certain distance 
into the legs ; sometimes, indeed, they proceed 
no farther than the haunches. The currents of 
blood in these vessels have not a uniform velo^ 
city, being accelerated by the impulsions they 
receive from the contractions of the dorsal 
vessel, which appears to be the prime agent in 
their motion. 

As the insect advances to maturity, and passes 
through its metamorphoses, considerable changes 
are observed to take place in the organization of 
the circHlating system, and in the energy of the 
function it performs. The vessels in the extreme 
parts, as in the tail, are gradually obliterated, 
and the circulalion in them, of course, ceases, the 
blood appearing to retire into the more internal 
parts. In the wings, on the other hand, where 
the developement proceeds rapidly, the circula- 
tion becomes more active ; and even after they 
have attained their ftiU size, and are yet in a 
soft state, the motion of the blood in the centre 
of all the nervures is distinctly visible:* but 
afterwards, as the wings become dry, it ceases 
there also, and is then confined to the vessels 

• These currents in the wing of the Semblis bilineata have 
heen described Hod-^Mineated by Carus, in the Acta Acad. Cecs. 
Leop. Carol. Nat. Cur. vol. xv. part ii- p. 9. 


of the trunk. In proportion as the insect ap- 
proaches to the completion of its developement, 
these latter vessels also, one after the other, shrink 
and disappear, till at length nothing which had 
once appertained to this system remains visible, 
except the dorsal vessel. But as we observe 
this vessel still continuing its pulsatory move- 
ments, we may fairly infer that they are designed 
to maintain some degree of obscure and imperfect 
circulation of the nutrient juices, through vessels, 
which may, in their contracted state, correspond- 
ing to the diminished demands of the system, have 
generally escaped detection. In confirmation of 
these views it may be stated, that several ob- 
servers have, at length, succeeded in tracing 
minute branches, proceeding in different direc- 
tions, from the dorsal vessel, and distributed 
to various organs. The division of the anterior 
part of the dorsal vessel into descending branches 
was noticed by Comparetti. Dug^s has observed 
a similar division of this vessel in the corselet of 
several species of Phalen^y and farther ramifica- 
tions in that of the Gryllus lineola : and Audouin 
has traced them in many of the Hymenoptera.^ 

* Annales des Sciences Naturelles, it. 308. 

The figures which follow (from 339 to 345) are represen- 
tations, of the natural size, of the dorsal vessel of the Sphinx 
ligustrij or Privet Hawk -moth, which has been dissected in its 
three different stages, with great care, by Mr. Newport, from 



The discovery of the circulation in insects, and 
of its varying energy at different periods of 

whose drawings these figures have been eBgraved, and to whom 
I am indebted also for the description which follows :— 

The dorsal vessel of this insect is an elongated and gradually 
tapering vessel^ extending from the hinder part of the abdomen, 
along the back, towards the head ; and furnished with valves, 



-nj^'lVI'T' IWX^ 



which correspond very nearly in their situation to the incisions of 
the body. During the changes of the insect from the larva to the 
imago state, it undergoes a slight modification of form. In 
every state it may be distinguished into two portions, a dorsal and 
an aortal. The dorsal portion, which is the one in which a pulsa- 
tion is chiefly observable, is furnished with distinct valves, is at- 
tached along the dorsal part of the body by lateral muscles, and 
has vessels which enter it laterally, pouring into it the circulating 
fluid, which is returning from the sides and inferior portions of 
the body. In the caterpillar, this portion of the dorsal vessel ex- 
tends from ^e twelfth to the anterior part of tbe fifth segment. 
It is furnished with eight double valves, which are formed, as 
Mr. Bowerbank has correctly described them in the Ephemerm 
mar^nala ; namely, the upper valve '' by a reflecting inwards 


growth, has elucidated many obscure points in 
the physiology of this important class. It ex- 

and upwards of the inner coat, or coats of the artery," (by which 
he means the dorsal vessel) *' and the under one by a contraction 
or projection of the like parts of a portion of the artery beneath, 
BO as to come within the grasp of the lower part of the valve 
above it." The whole vessel is made up of three coats, the two 
innermost of which, the lining, or serous, and the muscular, or 
principal portion of the vessel, constitute the reflected portions, or 
valves ; while the third, or outermost coat, which is exceedingly 
thin and delicate, is continued over the vessel nearly in a straight 
line, and does not appear at all to follow the reflexions of 
the other two. In the caterpillar, this portion of the vessel has 
eight pairs of small suspensory muscles, seen along the upper side 
of Fig. 339, which arise from th^ middle of the upper surface of 
each valve, and are continued back to be attached over the middle 
of the next valve : they seem to have considerable influence over 
the contractions of the valves. The Aortal, or anterior portion 
of the vessel, extends from the hinder part of the fourth segment 
to its termination and division into vessels, to be distributed to the 
head, which division takes place after it has passed the oesopha- 
gus, and at a point immediately beneath the supra*oesophageal 
ganglion, or brain of the insect. This portion of the vessel is 
much narrower than the dorsal, has no distinct valves, or muscles ; 
nor do any vessels enter it laterally ; but it is very delicate and 
transparent, and gradually diminishes in size from its commence* 
ment to its anterior termination. Its course, in the caterpillar, 
is immediately beneath the integument, along the fourth and 
third segments, till it arrives at the hinder parts of the second 
segment ; when it gradually descends upon the oesophagus, and, 
immediately behind the cerebral ganglion, gives off a pair of ex« 
ceedingly minute vessels. It then passes beneath the ganglion, 
and, in the front part of the head, is divided into several branches, 
as noticed by Mr. Newport in the anatomical description he has 
given of the nerves of this species of Sphinx : (Phil. Trans. 1832, 
p. 385.) These branches are best observed in the chrysalis (Fig. 


plains why insects, after they have attained their 
imago state, and the circulation is neaiiy oblite- 

339) : in all the stages they may be divided into three sets ; the 
first is given off immediately after the vessel has passed beneath 
the ganglion, and consists of two lateral trunks, the united capa- 
city of which is equal to about one-third of that of the aorta ; they 
descend, one on each side of the mouth, and are each divided 
into three branches. The second set .consists of tw6 pairs of 
branches, one gobg apparently to the tongue, the other to the 
antennee. The third set is formed by two branches, which pass 
upwards, and are the continuations of the aorta ; they divide into 
branches, and are lost in the integuments, and structures of the 
anterior part of the head. 

The pulsatory action of the dorsal vessel is continued along its 
whole course, and seems to terminate at the division of the vessel 
into branches. During the metamorphoses of the insect, this 
vessel becomes considerably shortened, but is stronger, and more 
consolidated in its structure. Its course is likewise altered ; from 
having, in the caterpillar (Fig. 339) passed along, nearly in 
a straight line, it begins, in the chrysalis (Fig. 340), to descend 
in the fifth segment, and to pass under what is to become the di- 
vision between the thorax and abdomen in the perfect insect. It 
then ascends in the fourth, segment, and descends again in the 
second, so that when the insect has attained its perfect form, 
(Fig. 341) its course is very tortuous. The vessels which enter 
it are situated in the abdomen, and pass in laterally among the 
muscles, chiefly at the anterior part of each segment or valve. 
Fig. 342 is a superior, or dorsal view of the same vessel, in the 
perfect state of the insect, which shows still more distinctly the 
vessels entering it laterally, intermixed with the lateral muscles. 
Fig. 343 is a magnified lateral view of the' anterior extremity of 
the dorsal vessel, conesponding to Fig. 341 ; and Fig. 344, a 
similarly magnified view of the same portion of the vessel seen 
from above, corresponding to Fig. 342. Fig. 345 shows the 
mode in which the valves are formed by a duplicature of the 
inner membrane in the perfect insect. 


rated, no longer increase in size, and require but 
little nourishment for the maintenance of life. 
This, however, is a state not calculated for so 
long a duration as that in which the develope- 
ment is advancing ; and accordingly, the period 
during which the insect remains in the imago 
condition is generally short, compared to that of 
the larva, where a l&rge supply of nutrim^it, and 
a rapid circulation of the fluids concur in main- 
taining the vital functions in full activity. Thus 
the Ephemera^ which Uves for two or three years 
in the larva state, generally perishes in the course 
of a few hours after it has acquired wings, and 
reached its perfect state of maturity. 

In proportion as the changes of form which 
the insect undergoes are less considerable, the 
evidences of a circulation become more distinct. 
Such is the case in many of the Apterous In- 
sects, composing the family of Myriapoda: in 
the Scolapendra morsitans (Linn.), for instance, 
Dug^s observed the dorsal vessel dividing into 
three large branches. 

Most of the tribes belonging to the class of 
Arachnida have likewise a dorsal vessel very 
analogous in its structure and situation to that of 
insects ; and as none of them undei^ any meta- 
morphosis, their vascular system admits of being 
Considerably developed, and becomes a per- 
mwent part of the organization. Fig. 346 
shows the dorsal vessel of the Aranea dames- 


tica^ or house spider, with some of the arte- 
rial trunks arising from it, lying 
^ imbedded in a thick mass of 

substance, having a similar oily 
character to that which is con- 
tained in large quantities in 
the principal cavities of insects* 
It is, in general, difficult to ob- 
tain a view of the circulation in 
the living spider, on account of 
the thick covering of hair which is spread over 
the body and the limbs ; but if a species, which 
has no hair, be selected for examination, we can 
see very distinctly, through the microscope, the 
motion of the blood in the vessels, by means of 
the globules it contains, both in the legs and in 
other parts, where it presents appearances very 
similar to those already described in the limbs 
of the larv8B of insects. 

A complete vascular circulation is established 
in all the animals which compose the class of 
Annelida ; the vessels being continuous through- 
out, and having sufficient power to propel the 
blood through the whole of its circuit. Great 
variety exists in the arrangement and distribu- 
tion of these vessels, depending on the form of 
the animal, the complication of its functions, 
and the extent of its powers. The first rudi- 
jtnent of a distinct system of circulating vessels, 
independent of the ramified tubes proceeding 


traced in them : an organization of this kind has 
already been noticed in T^Bnics^ 

It was, till very lately, the prevailing opinion 
among naturalists that all true insects are nou- 
rished by imbibition, and that there exists in 
their system no real vascular circulation of 
juices. In all the animals belonging to this 
class, and in every stage of their developement, 
there is found a tubular organ, called the dorsal 
vessdf extending the whole length of the bads:, 
and nearly of uniform diameter, except where it 
tapers at the two ends. It contains a fluid, 
which appears to be undulated backwards and 
forwards, by means of contractions and dilata- 
tions, occurring in succession in different parts 
of the tube; and it is also connected with 
transverse ligamentary bands, apparently con- 
taining muscular fibres, capable by their action 
of producing, or at least of influencing these pul- 
satory movements. An enlarged representation 
of the dorsal vessel of the Mdolantha vulgaris^ 
or common cockchaffer, isolated firom its attach-^ 
ments, is given in Fig. 333, showing the series 
of dilatations (v, v, v) which it usually presents 
in its course ; and in Fig. 334 the same vessel is 
exhibited in connexion with the ligamentary and 

* Page 83, in this volume; Fig. 247. The family of Pla- 
naruB present exceptions to this general rule : for many species 
possess a system of circulating vessels. See Dug^s, Annales 
des Sciences Naturelles; xv, 161. 


muscular apparatus which summuds it, seen 
irom the lower ude. In the last of these figures, 

A is the tapering prolongation of the tube, pro- 
ceeding towards the head of the insect ; v, one of 
the dilated portions, or Tentricles, as they have 
been called, of the dorsal part of the tube ; f, one 
of the small tendinous folds, to which the liga- 
mentary bands are attached ; and l is one of 
these bands, having a triangular, or, if considered 
as continuous with that on the other side of the 
vessel, a rhomboidal shape, and attached at b. 

838 THfi VITAL n^HCTIOHt. 

to the gupc^or aegmeatM of tlie abdomen. At i 
is seen a layer of the same fibres, whkh are 
partly ligamentous and partly muscular, passing 
underneath the dorsal vessel, and forming, in 
conjunction with the layer that passes above it, 
a sheath, which embraces and fixes that vessel 
in its place: these inferior layers have been 
removed from the other parts of the vessel, to 
allow the upper layers to be seen, as is the case 
at L. Fig. 335 gives a side view of the anterior 
extremity of the same vessel, showing the curve 
(a) which it describes as it bends downwards in 
its course towards the head. 

The fimction performed by the dorsal vessel, 
which, judging from the universal presence of 
this organ in insects, must be one of great im- 
portance in their economy, was long a profound 
mystery. Its analogy in structure and position 
to the dorsal vessels of the Arachnida and the 
Annelida, where it evidently communicates with 
channels of circulation, and exhibits movements 
of pulsaticm resembling those of insects, was 
a strong argument in favour of the opinion that 
it is the prime mover of a similar kind of circu-- 
laticMi ; but then, again, this hypothesis ap^ 
peared to be overturned by the fact that no 
vessels of any kind could be seen extending 
from it in any direction ; nor could any channels 
for the transmission of a circulating fluid be 
detected in any part of the foody. Those organs. 


which, in animals appajnently of an inferior mfikp 
ana most vascular, such as the stomach, the 
intestinal tuha, the eye, and other apparatus 
of the senses^ seemed to be constructed, aud 
to be nourished* by means totally different from 
those adopted in the former animals^ Although 
extr/emely minute rami&eations of air tubes stfe 
every where visible in the interior of insects, 
yet, neither Cuvier, nor any other anatomist, 
CQuld succeed, by the closest scrutiny, in de- 
tecting the least trace of blood vessels ; and the 
presumption, therefore, was, that none existed. 

But it still remained a question, if the dorsal 
vessel be not subservient to circulation, what 
is its real function? Marcel des Serres, who 
bestowed great pains in investigating this sub- 
ject, came to the conclusion that its use is to 
secrete the fatty matter, which is generally 
found in great abundance in the abdominal 
cavity, and which is accumulated particularly 
around the dorsal vessel. * A more attentive 
exanunation of the structure of the vessel itself 
brought to light a valvular apparatus, of which 
the only conceivable purpose is that oi deter- 
mining the motion of the contained fluid in one 
constant course ; a purpose necessarily incom- 
patible with its supposed alternate undulation 

* See his various papers in the M^moires du Museum d' Hiet. 
Nat. ; tom iv. and v. 


vessels are seen pursuing a slightly serpentine 

The tribe of Laimhrici, which includes the 
earth-worm, is distinguished from the annelida 
already noticed, by being more highly organized, 
and possessing a more extensive circulation, and 
a more complicated apparatus for the per- 
formance of this function. The greater extent 
of vascular ramifications appears to require in- 
creased powers for carrying the blood through 
the numerous and intricate passages it has to 
traverse; and these are obtained by means of 
muscular receptacles, capable, by their succes- 
sive contraction, of adding to the impulsive force 
with which the blood is driven into the trunks 
that distribute it so extensively. These muscu- 

* Dug^s repreBentfl the blood of this animal as Moving in 
different directions in the right and in the left lateral vessels ; 
generally backwards in the former, and forwards in the latter : 
at the same time that it moves backwards in the dorsal, and 
forwards in the abdominal vessel. In the communicating 
branches which pass transversely from one lateral vessel to the 
other, the blood flows from left to right in those situated in the 
anterior half of the body, and from right, to left in those of 
the posterior half: so that the plane in which its circuit is 
performed is horizontal, instead of vertical. It is curious to 
find an example of a similar transverse circulation, in the 
vegetable kingdom ; this has recently been observed by Mr. 
Solly and Mr. Varley, in a sprout of the Chara vulgaris, near 
the end of which the enclosed fluid revolves continually on 
its own axis, instead of following the ordinary course of ascent 
and descent along the sides of the cylindric cavity. — See Trans. 
of the Society of Arts, xlix, 180. 


lar appendages are globular or oval dilatations 
of some of the large vascular trunks, which bend 
round the sides of the anterior part of the body, 
and establish a free communication between the 
dorsal and th6 abdominal vessels. They are 
described by Dug^s as consisting, in the Lum- 
hricua gigas, of seven vessels on each side, form- 
ing a series of rounded dilatations, about twelve 
in number, resembhng a string of beads.* 

In the LMTobricus terrestris, or common earth- 
worm, there are only five pair of these vessels; 
they have been described and figured by Sir 
£. Hornet = ^ut the most fiill and accurate 
account of their structure has been given by 
Morren, in his splendid work on the anatomy of 
that animal.| Fig. .349, which is reduced from 

• They are termed by Dug^s, Vaisseaux moniliformes, ou 
dorto-abdomiTiaux.— Antilles des ScieQces Naturelles, xv, 299. 

t Philos. TniQsact. for 1817, p. 3 : and PI. iii. Fig. 4. 

t " De Lumbrici terrestris Hiatoria oatnralis, necnon Anatomia 
Tractatus." Qto. Bruxelles, 1829. 


his plates, represents these singular appendages 
to the vascular system of the earth-worm, sepa- 
rated from their attachments, and viewed in con- 
nexion only with the dorsal and abdominal trunks 
in which they terminate. The abdominal vessel, 
(a, a), on arriving near the oesophagus, is dilated, 
at the point b, into a globular bulb (c), which 
is followed, at equal intervals, by four others 
(c, c). From each of these bulbs, or ventri- 
cles^ as they are termed by Morren, a vessel (d) 
is sent off at right angles, on each side; this 
vessel also enlarges into several nearly globular 
dilatations (e), followed by a still larger, and 
more elongated oval receptacle (f), which com- 
pletes the semicircular sweep taken by the vessd 
in bedding round the sides of the body, in 
order to join the dorsal vessel (g, g), in which 
all the other four communicating vessels, pre- 
senting similar dilatations, terminate. Sir E. 
Home is of opinion that these dilated portions of 
the vessel are useful as reservoirs of blood, for 
supplying it in greater quantity to the neigh- 
bouring organs, as occasion may require: but 
Morren ascribes to them the more important 
office of accelerating, by their muscular action, 
the current of circulating blood. If the latter of 
these views be correct, which the strong pulsa- 
tions constantly visible in these bulbs render 
extremely probable, this structm'e would offer 
the first rudiments of the oi^an which, in all the 


superior classes of animals, perfomis so impor- 
tant an office in the circulation of the blood, 
namely, the heart: and this name, indeed, is 
given by Cuvier, Morren, and others, to these 
dilated portions of the vascular systems of the 
higher orders of Annelida.* 

Here, also, the statements of different anato- 
mists are at variance, with regard to the direc- 
tion taken by the blood while circulating in the 
vessels : Home and Dug^s represent it as pro- 
ceeding forwards in the dorsal, and backwards 
in the abdominal vessels; a course which im- 
plies its descent along the lateral communicating 
vessels just described ; while De Blainville and 
Morren ascribe to it a course precisely the 
reverse. Amidst these conflicting testimonies, 
it is extremely difficult to determine on which 
side the truth lies; and a suspicion will natu- 
rally arise, that the course of the blood in the 
vessels may not be at all times uniform, but may 
be liable to partial oscillations, or be even com- 
pletely reversed, by the operation of particular 
disturbing causes. 

The larger Crustacea possess a circulatory 
apparatus still more extensive and complete, 
accompanied by a corresponding increase in the 
energy of the vital functions. As we follow this 

* It 18 remarkable that the blood in most of the Annelida has 
a bri^^ht scarlet colour, and resembles in this respect the blood 
of vertebrated animals. 



system in the more highly organized tribes of 
this class, we find the powers of the dorsal 
vessel becoming more and more concentrated 
in its anterior extremity ; till in the Decapoda^ 
a family which comprehends the Lobster and 
the Crab, we find this part dilated into an oval 
or globular organ, with very mnteular coats, 
capable of vigorous contractions, propelling its 
contents with considerable force into the vessels, 
and therefore clearly entitled to the appellation 
of heart. The distinction between arteries and 
veins, which can scarcely be made with any 
precision in the systems of the inferior tribes, is 
here perfectly determined by the existence of 
this central organ of propulsion : for the vessels 
into which the blood is sent by its contractions, 
and which, ramifying extensively, distribute it to 
distant parts, are indisputably arteries; and con* 
versely, the vessels, which collect the blood itova 
all these parts, and bring it back to the heart, 
are as decidedly veiti^. The heart of the lobster 
is situated immediately under the carapace, or 
shell of the dorsal region of the thorax, di* 
rectly over the Stomach : its pulsations are very 
distinct, and are perfbrmed with great regularity. 
The importance of the heart, as the prime 
agent in the circulation, in<^reases as we advance 
to the higher classes of animals, whose more 
active and energetic functions require a con- 
tinual and rapid renewal of nutrient fluid, and 
render necessary the introduction of farther re- 


finonents into its structure. The supply of 
blood to the heart, being in a constant stream, 
produces a gradual dilatation of the cavity which 
receiveB it ; and the muscular fibres of that cavity 
are not excited to contraction, until they nre 
stretched to a certain poinL But in order effec- 
tually to drive the blood into every part of the 
arterial system, where it has great resistances 
to overcome, a considerable impulsive force is 
required, implying a sudden as well as powerful 
muscular action. This object is stained, in all 
vertebrated animals, by providing a second 
muscular cavity, termed a ventricle, into which 
the first cavity, or auricle, throws the blood it has 
receaved from the veins, with a sudden impulse ; 
and thus the ventricle, being rapidly distended, 
is excited to a much more quick and forcible 

contraction than tiie auricle, and propels the 
blood it contains into the artery, with an impetus 


inccMDparably greater than could have resulted 
from the action of the auricle alooe. Fig. 350 
represents the heart with its two cavities ; d being 
the auricle, and b the ventricle ; t<^ether with 
the main trunks of the veins (c, c,) which con- 
vey the blood into the auricle ; and those of the 
arteries (a), which receive it from the ventricle, 
for distribution over the whole system. 

The force of contraction in the principal 
cavity of the heart being thus increased, it 
becomes necessary to provide additional secu- 
rities against the retrograde motion of its fluid 
contents. Valves are accordingly interposed 
between the auricle and ventricle; and great 
refinement of mechanism is displayed in their 
constructioD. Fig. 351 represents their appear- 

ance (at v) when die cavities, both of the auricle 
(d), and the ventricle (e) are lud open : c, c, as 
before, being the upper and lower venee cavse, 
and A, the main trunk of the aorta. These 


valves are coroposed of two loose membranes, 
the fixed edges of which are attached circularly 
to the aperture of communication between the 
cayities, and their loose edges project into the 
ventricle; so that they perform the office of 
flood*gates, allowing a ^e passage to the blood 
when it is impelled into the ventricle, and being 
pushed back the moment the ventricle contracts ; 
in which latter case they concur in accurately 
closing the aperture, and preventing the return 
of a single drop into the auricle. These valves 
being attached to a wide circular aperture, it is 
necessary that they should be restrained from 
inverting themselves into the auricle, at each 
contraction of the ventricle. For this purpose 
there are provided slender ligaments (which are 
seen in Fig. 35 1), fixed by one end to the edge 
of the valve, and by the other to some part of 
the inner surface of the ventricle, so that the 
valve is always kept within the cavity of the 
latter. In the auricle, the same purpose is 
answered by the oblique direction in which the 
veins enter it. 

The arteries themselves, especially the main 
trunk of the aorta, as it issues from the heart, are 
muscular, and when suddenly distended, contract 
upon their contents. It was necessary, therefore, 
to provide means for preventing any reflux of 
Uood into the ventricle during their contraction ; 
and for this purpose a set of valves (r, Fig. 351) 


is placed at the beginning of these tubeei, where 
they arise from the ventricle. These valves con- 
sist usually of three membranes, which have the 
form of a cr^cent, and are capable of closing the 
passage so accurately, that not a drop of blood 
can pass between them.* 

In order to convey a more clear idea of the 
course of the blood in the circulatory system, I 
have drawn the diagram, 
Fig. 3.32, exhibiting the 
general arrangement of 
its component parts. Hie 
' main arterial trunk, or 
Aorta (a), while proceed- 
ing in its course, gives off 
numerous branches (b), 
which divide and subdi- 
vide, till the ramificati<ms 
(p) MTive at an extreme 
degree of minuteness; 
and they are finally distributed to every organ, 
and to the remotest extremities of the body. They 
frequently, during their course, comiftnnicate 
with one another, or anastomosei as it is termed, by 
collateral branches, so as to provide against in- 

■ In the artery of the ihark, and other cartilagmoQS fiafaei, 
where the Action of the vessel is very powerful, these ralvea are 
much more numerous, aud arranged in rows, occupying several 
parts of the artery. Additional valves are also met with in other 
fishes at the branching of large arteries. 


tenaptiiHid to the circulation, whieh might arise 
£rom accidental obstractions in any particular 
branches of this extended system of canals. 
The minutest vessels (p), which in incalculable 
numbers, pervade every part of the frame, are 
named, from their being finer than hairs, capil'- 
fartf vessels. 

After the blood, thus transmitted to the differ- 
ent parts of the body by the arteries, has supplied 
them with the nourishment they require, it is 
conveyed back to the heart by the veins, which, 
eomm^icing from the extreme ramifications of 
the arteries, bend back again in a course di- 
rected towards the heart. The smaller branches 
j(»n in succession to fbnn larger and larger 
trunks, till they are at length all united into one 
or two main pipes, called the VetuB cav^e^ (c), 
which pour their accumulated torrent of blood 
into the general reservoir, the heart; entering 
first into the auricle (d), and thence being carried 
forward into tjbe ventricle (e), which again pro- 
pels it through the Aorta. The veins are larger 
and more numerous than the arteries, and may 
be compared to rivers, which collecting all the 
water that is not imbibed by the soil, and recon- 
veying it into its general receptacle, the ocean, 
perform an analogous office in the economy 
of the earth. 

The communications of the capillary arteries 
with the veins are beautifully seen, under the 


microscope, in the transparent membranes of 
frogs or fishes. The splendid spectacle, thus 
brought within the cognizance of our senses, of 
unceasing activity in the minutest filaments of 
the animal frame, and of the rapid transit of 
streams of fluid, bearing along with them minute 
particles, which appear to be pressing forwards, 
like the passengers in the streets of a crowded 
city, through multitudes of narrow and Mdnding 
passages, can never fail, when first beheld, to 
fill the mind with astonishment*; a feeling, 
which must be exalted to the highest admiration 
on reflecting that what we there behold is at all 
times going on within us, during the whole 
period of our lives, in every, even the minutest 
portion of our frame. How inadequate, then, 
must be any ideas we are capable of forming 
of the incalculable number of movements and of 
actions, which are conducted in the living sys- 
tem ; and how infinite must be the prescience 
and the wisdom, by which these multifarious and 
complicated operations were so deeply planned^ 
and so harmoniously adjusted ! 

* Lewenhoeck, speaking of the delight he experienced on 
viewing the circulation of the blood in tadpoles, uses the follow- 
ing expressions. " This pleasure has oftentimes been so recrea- 
ting to me, that I do not believe that all the pleasure of fountains, 
or water-works, either natural or made by art, could have 
pleased my sight so well, as the view of these creatures has 
given me.** — Phil. Trans, xxii. 453. 



§ 3. Respiratory Circulation. 

The object of the circulation is not merely to 
distribute the blood through the general system 
of the body ; it has also another and very im- 
portant office to perform. The blood undergoes, 
in the course of its circulation, considerable 
changes, both in its colour and its chemical 
composition. The healthy blood transmitted by 
the arteries is of a bright scarlet hue; that 
brought back by the veins is of a dark purple, 
from its containing an excess of carbon, and 
is consequently unfit to be again circulated. 
Whenever, from some derangement in the func- 
tions, this dark blood finds its way* into the 
arteries, it acts as a poison on every organ which 
it reaches, and would soon, if it continued to 
circulate, destroy life. Hence it is necessary 
that the blood which returns by the veins should 
undergo purification, by exposure either to the 
air itself, or to a fluid containing air, for the 
purpc«e of restoring and preserving its salutary 
qualities. The heart and vascular system have 
therefore the additional task assigned them of 
conveying the vitiated venous blood to certain 
organs, where it may have access to the air, and 
receive its vivifying influence ; and to this office 
a distinct set of arteries and veins is appro- 


priated, constituting a distinct circulation. This 
I have endeavoured to illus- 
trate by the digram, Fig. 
363, where d represents the 
auricle, and b the ventricLe 
of the heart ; and a and c, 
A the main arterial and t^ioub 
I trunks ; and where the two 
circulations are, for the sake 
of distinctness, supposed to 
be separated from one ano- 
tlier, so that the two syst^ns 
of vessels may occupy dif- 
ferent parts of the diagram. 
The vessels which pervade the body generally 
(b), and are subservient to nutrUion, b^ng to 
what is termed the greater, or systemic circula- 
tion : those which circulate the blood through the 
respiratory organs, (a), for the purpose of aera- 
ti<Mi, compose the system of the lesser, or reipt- 
Tatory circulation. 

Few subjects in Physiology present a field 
of greater interest than the comparison of the 
modes in which these two great functions are, 
in all tbe various classes of animals, exacdy 
adjusted to each other. So intimately Are the 
<a^ans of circulation related to those which 
distribute the blood to the respiratory o^^ans, 
that ve never can form a dear \<AeA of the first, 
without a close reference to the last «f these 


syBtems. While describing the sereial plans 
of circulation presented to us by the diffident 
classes, I shall be obliged, therefore, to assume 
both the necessity oi the function of respiration, 
and of a provision of certain organs for the 
reception of air, either in its gaseous form, or 
as it is contained in water, where the blood 
may be subjected to its action. It is necessary, 
also, to state that the organs for receiving atmos- 
pheric air in its gaseous state are either lungs, 
or pulmanaiy cavities, while those which are 
constructed for aquatic respiration are tenned 
giUs, or branchiae ; the arteries and the veins 
which carry on this respiratory circulation, being 
termed pulmonary, or branchial, according as 
they relate to the one or the other description 
Jitf respiratory organs. 

In /many animals it is only a part of the cir- 
culating blood which undergoes aeration; the 
puhnonary or branchial arteries and veins being 
mer^y branches <^ the general system of blood 
vessels: so that in this case, which is that 
represented in the preceding fi^re (353), the 
lesser drculatian is indkiided as a part of the 
general circulation. But in all the higher classes 
the whole of the blood is, in some part of its 
circuit, sul^ected to the influence of the air; 
the pulmonary, being then distinct from the 
systemic drculation. In the Annelida, for in- 
iitance, the vanse cvirae, which bring back the 


blood from the system, unite to form one or 
more vessels, which then assume the function 
of arteries, subdividing and ramifying upon the 
branchial organs ; after this the blood is again 
collected by the branchial veins, which unite 
into one trunk to form the arteries of the sys* 
temic circulation. 

Most insects, especially when arrived at the 
advanced stages of their developement, have too 
imperfect a circulation to elSTect the thorough 
aeration of the blood : and indeed a greater part 
of that fluid is not contained within the vascular 
system, but permeates the cavities and cellular 
texture of the body. It will be seen, when I 
come to treat of respiration, that the same object 
is accomplished by means totally independent of 
the circulatory apparatus ; namely, by a system 
of air-tubes, distributed over every part of the 
body. But an apparatus of this kind is not. 
required in those Arachnida, where the circulation 
is vigorous, and continues during the whole of 
life : here, then, we again meet with a pulmonary 
as well as a systemic circulation, in conjunction 
with internal cavities for the reception of air. 

In the Crustacea the circulation is conducted 
on the same general plan as in the Annelida ; the 
blood from every part of the body being collected 
by the Venae Cavse, which are exceedingly capa- 
cious, and extend, on each side, along the lower 
surface of the abdomen . They send out branches, 


which distribute the blood to the gills ; but these 
branches, at their origin, suddenly dilate, so as 
to fonn lai^ receptacles, which are called 
sinuses, where the blood is allowed to accumu- 
late, and where, by the muscularity of the ex- 
panded coats of the vessels, it receives an addi- 
tional force of propulsion. From the branchiae 
the blood is returned by another set of veins 
to the elongated heart formerly described, and 
propelled by that or- 
gan into the systemic 
arteries. Fig. 354 
shows the relative si- 
tuation of these ves- 
sels, when isolated 
and viewed from be- 
hind, in the Maja squinado. c, c, are the vente 
cave ; e, e, the venous sinuses above-mentioned ; 
F, F, are the branchial arteries \ g, the gills, or 
branchiee ; and i, i, the branchial veins termina- 
ting in the heart i,.* 

In the Mollusca, the heart acquires greatef 
size, compared with the other o^ans, and exerts 
a proportionally greater influence as the prime 
mover in the circulation. In the developement 
of its structure, in the different orders of this 

* A minute account of the organs of circulation in the Crusta- 
cea is given by Audouin and Milne Edwards, in the Annates des 
Sdences Natnielles, xi, 283 and 352, from which worli the above 
6gare ii taken. 


class, a beautiful gradatioii may be percebred : 
the JBranchiapoda having two hearts, one placed 
upon each of the two lateral trunks of the 
branchial veins ; the Gastef^apoda having a lungle 
heart, :l^imished with an auricle ; and the Ace- 
phala being provided with a heart, which has 
a single ventricle, but two auricles, corresponding 
to the two trunks of the branchial veins.* 

The most remarkable variety of structure is 
that exhibited by the Cephalopoda. We have 
already seen, in the Crustacea, dilatations of the 
venae cavae, at the origin of the branchial arte- 
ries : but in the Nautilus the dilatations of the 
branchial veins are of such a size, as to be almost 
entitled to the appellation of auricles. The 
Sepiuj in whose highly organized system there is 
required great additional power to propel tbie 
blood with sufficient force through the giltei is 
provided with a large and complicated branchiid 
apparatus ; and the requisite power is snpplt^Ml 
by two additional hearts, situated on the venpe 
cavsB, of which they appear as if they were 
dilatations, immediately before the branphial 
arteries are sent off*t They are shown at Bi Ei 
Fig* 355, which. represents this part of the vaa- 

* A peat iiaraberofbhrahr^MoUuscai exhibit the singQkrp^ 
culiarity of the lower portion of the intestiaal tube trayersiing 
through the cavity of the heart. 

t These veins are surrounded by a great number of blind 
pouches, which have the appearattce of a firing^ ; the use of tlK3 
singular structure is unknown. 


cular system of the Loligo, detached from the 
surrounding parts ; the course of the blood being 

indicated by arrows, c is one of the three 
trunks constituting the renee cavse, proceeding 
from above, dividing into two branches as it de- 
8cends,'and terminating, conjointly with the two 
venous trunks (d), which are coming from below, 
into the lateral or branchial hearts (b,e), already 
mentioned. Thence Ute blood is conveyed by the 
branchial arteries, (f,f), on each side, to the gills 
(g), and returned, by the branchial veins (i), to 
the lai^e central, or systemic heart (l), which 
again distributes it, by means of the systemic ar- 
teries, to every part of the body. The cuttle-fish 
tribe is the only one thus furnished with three 
distinct hearts for carrying on a double circula- 
tion ; none of these hearts are furnished with 


The lemaikaUe distribution of the 
poirers which giTe an impolse to the circulating 
flnids, met with in the Sepia, constitntes a 8t^ 
in the transition firom 3Iollusca to Fishes. In 
this hitter class of animals, the two lateral hearts 
hare united into a single central heart, while the 
aortic heart has eikUnij disappeared ; and thus 
the pontion of the heart with respect to the two 
circulations is just the rererse of that which it 

has in the invertebrated 
classes. The plan in Fishes 
is shown in the diagram. 
Fig. 356, where the cen- 
tral organs are seen to con- 
sist of four cavities, c, o, e, 
F, opening successively the 
one into the other. The 
heart bdongs exclusively 
to the gills ; and there pro* 
ceeds from it, not the aorta^ 
but the trunk of those 
branchial arteries (f), which convey the whole of 
the blood to the respiratory organs (o, h). This 
blood, after being there aerated, is collected by 
the branchial veins (i), which unite into a single 
trunk (a), passing down the back, and perform- 
ing, without any intermediate heart, the c^ce of 
an aorta ; that is, it divides into innumerable 
branches, and distributes the blood to every part 


of the system.* The blood is then reconveyed to 
the heart by the orduiary veins, which form a large 
vena cava (c). This vein is generally consider- 
ably dilated at its termination, or just before it 
opens into the auricle, constituting what has 
been termed a ven&iis sinus (s). This, then, is 
followed by the auricle (d) and the ventricle (,e) ; 
but, besides these cavities, there is also a fourth 
(f), formed by a dilatation of the beginning of 
the branchial artery, and termed the bulbus arte-- 
riasusy contributing, doubtless, to augment the 
impetus with which the blood is sent into the 
branchial arteries. 

The circulation in Reptiles is not double, like 
that of fishes; for only a part of the blood 
is brought under the influence of the air in 
the pulmonary organs. All the animals belong- 
ing to this class are cold-blooded, sluggish, and 
inert; they subsist upon a scanty allowance of 
food, and are astonishingly tenacious of life. 
The simplest form in which we meet with this 
mode of circulation is in the Batrachia; it is 

* The caudal branch of the aorta is protected by the roots of 
the laferior spinous processes, joining to form arches through which 
it passes ; and frequently the artery is contained in a bony chan- 
Defy formed by the bodies of the Yertebrse, which effectually se- 
cares k Aom all external pressure. In the sturgeon even the ab- 
dominal fiorta is thus protected, being entirely concealed within 
this bony canal. 

VOL. II. 'J' 

274 l^E VITAL FVNcnOMS. 

shown in the diagram. Fig, 357. The heart (rf 
the Frog, for example, may be considered as 
consiBting of a single ven- 
tricle (e), and a single av 
] ricle (d) * From liie former 

there proceeds one great ar* 
terial tmnk, which is pro^ 
periy the aorta. This aorta 
soon divides into two trunks, 
which , after sendii^ branches 
to the head and neck, bend 
. downwards (as is seen at 

o, p), and nnite to form a 
single trunk (a), which is 
the descending aorta. From this vessel pro- 
ceed all the arteries which are distributed to 
the trunk and to the limbs, and which are 
represented as situated at b : these arterial rami- 
fications are continued into the great venous 
trunks, which, as usual, constitute the vmee 
cavsB (c), and terminate in the auricle (d). 

From each of the trunks which arise from the 
primary division of the aorta, there proceed the 

* Dr. Davy has observed that although the auriele appears 
■ingle; when viewed externally, its cavity ia in reality dMded 
into two compartments by a traniparent membnuioiu partitioD, 
in which some miuculaf fibres are apparent : these commnnicate 
with the cavity of the ventricle by a common opening, provided 
with three semilunar valves. Edin. Phil. Journal; xix, 161. - 


smidl arteries (f), which are distributed to the 
longs (h)» and convey to those organs a part 
only of the mass of circulating blood. To these 
pulmonary arteries there correspond a set of 
veins, uniting in the trunks (i), which bring 
bock the aerated blood to the auricle of the 
heart (d), where it is mixed with the blood 
which has returned by thfe venee cavee (c), from 
the general circulation. Thus the blood i& only 
partially aerated, in consequence of the lesser 
eirculadon being here only a branch of the 

Nothing is more curious or beautiful than the 
mode in which, during the transformations of 
this animal, Nature conducts the gradual tran- 
sition of the branchial pirculation of the tadpole, 
into the pulmonary circulation of the frog. In 
the fonner, the respiratory oigans are constructed 
on the model of those of fishes, and respiration 
is performed in the same manner as in that class 
of animals : the heart is consequently essentially 
branchial, sending the whole of its blood to the 
gills, the veins returning from which (describing 
the course marked by tlie dotted lines m, n, in 
the diagram), unite, as in fishes, to fonn the 
descending aorta. As the lungs develope, small 
arterial branches, arising from the aorta, are 
distributed to those organs, and in proportion as 
these arteries enlarge, the branchial arteries 


diminish ; until, on their becoming entirely ob- 
literatedy the course of the blood is wholly 
diverted from them, and flows through the 
enlarged lateral trunks (o, p,) of which the 
junction constitutes the descending aorta. This 
latter vessel now receives the whole of its blood 
directly from the heart; which, from being 
originally a branchial, has become a systemic 

The heart of the Chelonian reptiles, such as thei 
ordinary species of Tortoises and Turtles, has 
two distinct auricles ; the one, receiving the blood 
from the pulmonary veins ; the other, from those 
of the body generally ; so that the mixture of 
aerated and vitiated blood takes place, not in the 
auricle, but in the ventricle itself. When all the 
cavities are distended with blood, the two auricles 
being nearly of the same size as the ventricle, 
the whole has the appearance of a union of 
three hearts. The circulatory system of the 
Ophidia is constructed on a plan very similar 
to that of the Chdonia. 

In the Saurian reptiles, the structure becomes 
again more complicated. In the Champion each 
auricle of the heart has a large venous sinus, 
appearing like two supplementary auricles.* 
The heart of the Crocodile has not only two 

* Houston; Trans. Roy. Irish Acad, xv, 189. 


auricles, but its ventricle is divided, by two par- 
titions, into three chambers : each of the par- 
titions is perforated to allow of a free communi- 
cation between the chambers ; and the passages 
are so adjusted as to determine the current of 
aerated blood, returning from the lungs, into 
those arteries, more especially, which supply the 
head and the muscles of the limbs ; while the 
vitiated blood is made again to circulate through 
the arteries of the viscera.* 

It is in warm-blooded animals that the two 
offices of the circulation are most efficiently per- 
formed ; for the whole of the blood passes 
alternately through the greater and the lesser 
circulations, and a complete apparatus is pro- 

* It would appear, from this arrangement of the yessels, that 
the brain, or central organ of the nervous system, requires, 
more than any other part, a supply of oxygenated blood for the 
due performance of its functions. The curious provision which 
is made for sending this partial supply of blood of a particulaf 
qualitj m the larger kinds of reptiles, such as the Crocodile^ 
has been pointed out by many anatomists ; but has been lately 
investigated more particularly by M. Martin St. Ange. (See 
the Report of O. St. Hilaire, Revue M^dicale, for April, 1833). 
It is found that in these animals, as well as in the Chelonia, a 
partial respiratory system is provided for by the admission, 
through two canals opening externally, of aerated water into 
the cavity of the abdomen, where it may act upon the blood 
which is circulating in the vessels. Traces of canals of this 
description are also met with in some of the higher classes of 
vertebrated animals, as, for instance, among the Mammalia, in 
(he Monoiremata and the Marsupialia. 


Tided for each. Tfaere are, in fact, two hearts, 
the one on the left side impelHng the Uobd 
through the gresAsr, or systemic circulatioii ; 
the other, on the right side, appn^riated to the 
lesser, or puhaonary circulation. The annexed 
diagram (Fig. .369), il- 
lustrates the plan of the 
circulation in wano.- 
blooded animals. From 
the left ventricle (l) the 
blood is propelled into 
the aorta (a), to be dif- 
fused through the arte- 
ries of the system (b) to 
erery part, and pene- 
trating into all the capil- 
lary Vessels; thence it 
is returned by the veins, through the Tense cave 
(c), to the right auricle (d), which delivers it 
into the right ventricle (e). This right ventricle 
impels the blood, thus received, through the 
pulmonary arteries (f), into the lungs (at h), 
where it is aerated, and whence it is reconveyed 
by the pulmonary veins (i), into the left auricle 
(r), which immediately pours it into the left 
ventricle (l), the point from whence we had 
set out. 

Both the right and the left heart have their 
respective auricles and ventricles ; but they are 


all united in one envelope, so as to compose 
in appearance but a single oigan:* still, how- 
ever, the right and left cavities are kept per- 
fectly distinct ftvm one another, and are sepa- 
rated by thick partitions, allowing of no direct 
teinamission of fluid from the one side to the 
other. These two hearts may therefore be com- 
pared to two sets of chambers under the same 
roof, having each their respective entrances 
and exits, with a party-wall of separation be- 
tw^n them. This junction of the two hearts 
is conducive to their mutual strength : for the 
fibres of each intermix and even co-operate in 
their actions, and both circulations are carried 
on at the same time ; that is, both ventricles 
contract or close at the same instant ; and the 
same applies to the auricles. The blood which 

* A remarkable exceptioD to this geaeral law of conaolidatioa 
occurs in the heart of the Du- 
gon0, TepreBent«d in Fig. 360. 
in which it may be seen that 
the two ventitcles, t. and l, are 
almost entirely detached from 
each other. In this fi^re, which 
is taken from the Philotophical 
TraniactioaB for 1820, o is the 
systemic auricle, k the right or 
pulmonary ventricle, f the pul- 
monary artery, K the left or 
pulmonary auricle, l the left 
or lystemic ventricle, and a the aorta. 


has just returned from the body, and that from 
the lungs, the former by the venae cavoe, the 
latter by the pulmonary veins, fill their respec- 
tive auricles at the same instant ; and both 
auricles, contracting at the same moment, dis- 
charge their c(MiteDt8 simultaneously into their 
respective ventricles. In the like manner, at 
the moment when the left ventricle is propelling 
its aerated blood into the aorta, for the purposes 
of general nutrition, the right ventricle is like- 
wise driving the vitiated blood into the pul- 
monary artery, in order that it may be purified 
by the influence of the air. Thus the same 
blood which, during the interval of one pulsation, 
was circulating through the lungs, is, in the 
next, circulating through the body ; and thus 
do the contractions of the veins, auricles, ven^ 
tricles, and arteries all concur in the same 
general end, and establish the most beautiful 
and perfect harmony of action.* 

* Evidence is afforded of the human conformation being 
expressly adapted to the erect position of the body by the 
position of the heart, as compared with quadrupeds ; for in the 
latter, the heart is placed directly in the middle of the chest, 
with the point towards the abdomen, and not occupying any 
portion of the diaphragm ; but in man, the heart lies obliquely 
on the diaphragm, with the apex turned towards the left aide. 


^ 4* DistrifmHon of Stood- vessels. 

In the distribution of the arteries in the animal 
syBtem^ we meet with numberless proofs of wise 
and provident arrangement. The great trunks 
of both arteries and reins, which carry on the 
circulation in the limbs, are conducted always 
on the interior sides, and along the interior 
angles of the joints, and generally seek the 
protection of the adjacent bones. Grooves are 
formed in many of the bones, where arteries 
are lodged, with the evident intention of afford- 
ing them a more secure passage. Thus the 
principal arteries which supply the muscles of 
the chest, proceed along the lower edges of 
the ribs, in deep furrows formed for their pro- 
tection. Arteries are often still more effectually 
guarded against injury or obstruction by pass- 
ing through complete tubes of solid bone. An 
inrtance occurs in the arteries supplying the 
teeth, which pass along a channel in the lower 
jaw, excavated through the whole length of the 
bone. The aorta in fishes, after having supplied 
arteries to the viscera of the abdomen, is con- 
tinued to the tail, and passes through a channel, 
formed by bony processes from the vertebrae ; 
and the same kind of protectjipn is afforded 
to the corresponding artery in the Cetacea. In 


the fore leg of the laon, which is employed 
in actions of prodigious strength, the artery, 
without some especial provision, would have 
been in danger of being compressed by the 
violent contractions of the muscles: in order, 
therefore, to guard against this inconv^ence, it 
is made to pass through a perforation in the 
bone itsdf, where it is completely secure from 

The energy of every function is regulated 
in a great measure by the quantify of blood 
which the organs ex^cising that function re- 
ceive. The muscles employed in the mofit 
vigorous actions are always found to receive 
the largest share of blood. It is commonly 
observed that the right fore leg of quadrupeds, 
as well as the right arm in man, is stronger 
than the left. Much of this superior strength 
is, no doubt, the result of education ; the right 
arm being habitually more used than the lef^. 
But still the different mode in which the arteries 
are distributed to the two aims constitutes a 
natural source of inequaUfy. The artery sup^ 
plying the right arm with blood is the first 
which arises from the aorta, and it proceed^ 
in a more direct course from the heart than 
the artery of the left arm, which has its 
origin in common with the artery of that £adei 

* In like manner the coffiu bone of the Hone is perforated 
for the safe conTeyaoee of the aiterioB going to tlie foot. 


of the head. Hence it has been inferred that 
the right arm is originally better supplied with 
nonrishment than the left. It may be allied, 
in confirmation of this view, that in birds, where 
any inequatity in the actions of the two wings 
would have disturbed the regularity of flight, 
the aorta, when it has arriyed at' the centre of 
the chest, divides with perfect equality into two 
branches, so that both wings receive precisely 
the same quantity oi blood ; and the muscles, 
being thus equally nourished, preserve that 
equaKty <rf strength, which their fimction rigidly 

When a large quantity of blood is wanted in 
any particular organ, and yet the force with 
which it would arrive, if sent immediately by 
large arteries, might injure the texture of that 
organ, contrivances are adopted for diminishing 
its impetus, either by making the arteries pursue 
very winding and circuitous paths, or by sub- 
dividing them, before they reach their destination, 
into a great number of smaller arteries. The 
delicate texture of the brain, for instance, would 
be greatly injured by the blood being impelled 
with any considerable force against the sides of 
the vessels which are distributed to it ; and yet 
a very laige supply of blood is required by that 
oigan for the due performance of its functions. ' 
Accordingly we find that all the arteries which 
go to the iNrain are very tortuous in their course ; 


every flexure tending considerably to diminish 
the force of the current of blood. 

In animals that graze, and keep their heads 
for a long time in a dependent position, the 
danger from an excessive impetus in the blood 
flowing towards the head is much greater than 
in other animals; and we find that an ex- 
traordinary provision is made to obviate this 
danger. The arteries which supply the brain, 
on their entrance into the basis of the skull, 
suddenly divide into a great number of mi- 
nute branches, forming a complicated net-work 
of vessels, an arrangement which, on the well 
known principles of hydraulics, must greatly 
check the velocity of the blood conducted 
through them. That such is the real purpose 
of this structure is evident from the branches 
afterwards uniting mto larger trunks when they 
have entered the brain, through the substance of 
which they are then distributed exactly as in 
other animals, where no such previous sub- 
division takes place. 

In the Sradypus tridactt/luSf or great Ame- 
rican Sloth, an animal remarkable for the slow- 
ness of its movements, a plan somewhat ana- 
logous to the former is adopted in the structure 
of the arteries of the limbs. These arteries, at 
their entrance into both the upper and lower ex- 
tremities, suddenly divide into a great number 
of cylindric vessels of equal size, communicating 


in various places by collateral branches. These 
curiously subdivided arteries are exclusively 
distributed to the muscles of the limbs ; for all 
the. other arteries of the body branch off in the 
usual manner. This structure, which was dis- 
covered by Sir A. Carlisle,* is not confined to 
the Sloth, but is met with in other animals, as 
the Lemur tardigradus^ and the Lemur hris^ 
which resemble the sloth in the extreme slug- 
g^hness of their movements. It is extremely 
probable, therefore, that this peculiarity in the 
muscular power results from, or is at least in 
some way connected with this remarkable struc- 
ture in the arteries. In the Lion, and some other 
beasts of prey, a similar construction is adopted 
in the arteries of the head, probably with a view 
to confer a power of more permanent coiitrac-^ 
tion in the muscles of the jaws for holding a 
strong animal, such as a buffalo, and carrying it 
to a distance. 

That we may form an adequate conception 
of the immense power of the ventricle, or prime 
mover in the circulation of the blood, we have 
but to reflect on the numerous obstacles im- 
peding its passage through the arterial system. 
There is, first, the natural elasticity of the 
coats of the arteries, which must be overcome 
before any blood can enter them. Secondly, 

• Phil. Trans, for 1800, p. 98, and for 1804, p. 17. 


the arteries are, in most places, so connected 
with many heavy parts of the body, that A&r 
dilatation cannot be effected without, at the same 
time, communicating motion to them. Thus, 
when we sit cross-le^ed, the pulsation of the 
artery in the ham, which is pressed upon the 
knee of the other leg, is sufficiently strong to 
raise the whole leg and foot, at each beat of the 
pulse. If we consider the great weight of the 
leg, and reflect upon the length of the lever by 
which that weight acts, we shall be convinced of 
the prodigious force which is actually exerted by 
the current of blood in the artery in thus raising 
the whole limb. Thirdly, the winding course, 
which the blood is forced to take, in following 
all the oblique and serpentine flexures of the 
arteries, must greatly impede its motion. But 
notwithstanding these numerous and powerful 
impediments, the force of the heart is so great, 
that, in. defiance of all obstacles or causes of 
retardation, it drives the blood with imm^ise ve- 
locity into the aorta. The ventricle of the human 
heart does not contain more than an ounce of 
blood, and it contracts at least seventy tunes in 
a minute ; so that above three hundred pounds of 
blood are passing through this oi^an during 
every hour that we live. " Consider," says Paley, 
'' what an affair this is when we come to very 
large animals. The aorta of a whale is laiger in 
the bore than the main pipe of the water-works 


at London Bridge ; and the water roaring in its 
passage through that pipe is inferior in im- 
petus and velocity to the blood gushing through 
the whale's heart. An anatomist who under- 
stood the structure of the heart, might say b^ore- 
hand that it would play ; but he would expect, 
from the complexity of its mechanism, and the 
delicacy of many of its parts, that it should always 
be liable to derangement, or that it would soon 
work itself out. Yet shall this wonderful ma- 
dbine go on, night and day, for eighty years 
together, at the rate of a himdred thousand 
studies every twenty-four hours, having at every 
stroke a great resistance to overcome, and shall 
continue this action, for this length of time, 
without disorder and without weariness. To 
those who venture their lives in a ship, it has 
often been said that there is only a plank be- 
tween them and destruction ; but in the body, 
and especially in the arterial system, there is 
in many parts only a membrane, a ddn, a 
thread." Yet how well has every part been 
guarded from injury: how providentially have 
accidents been anticipated: how skilfully has 
danger been averted ! 

The impulse which the heart, by its powerful 
contraction, gives to the blood, is nearly ex- 
pended by the time it has reached the veins: 
nature has accordingly furnished them with 
numerous valves, all opening, of course, in a 


directi<m towards tbe heart ; so that, as long v 
the blood proceeds in its natural course, it meets 
with no impediment ; while a retrograde motion 
is effectually prevented. Hence external pre6» 
sure, occasionally applied to the veins, aaemts in 
promoting the flow of Uood to- 
wards the heart ; and hence the 
effects of exercise in accderating 
the circulation. Valves are more 
especially provided in the veins 
which pass over the muscles of the 
extremities, or which run immor 
diatdy beneath the skin; whUe 
they are not found in the more 
internal veins belonging to the 
viscera, which are less exposed to nneqmi 
pressure. These valves are delineated in Fi^. 
365, which represents the interior oi one <rf the 
large veins. 

The situation and structure of tbe valves be- 
longing to the hydraulic apparatus of the cizcu- 
latioa ftimish as unequivocal proo& of design as 
any that can be adduced. It was the observa- 
tion of these valves that first suggested to the 
mind of Harvey the train of reflexions which led 
him to the discovery of the real course of the 
blood in the veins, the arteries and the heart. 
This great discovery was one of the eariiest 
fniits of the active and rational spirit of inquiry, 
which at the era of Bacon's writings, was be- 


giiming to awaken the human mind from its 
long night of slumber, and to dissipate the 
darkness which had, for so many ages, over- 
shadowed the regions of philosophy and science. 
We cannot but feel a pride, as Englishmen, in 
the recollection, that a discovery of such vast 
importance as that of the circulation of the 
blood, which has led to all the modem improve- 
ments in the medical art, was made by our own 
countryman, whose name will for ever live in the 
annals of our race) as one of its most distin- 
guished benefactors. The consideration, also, 
that it had its source in the study of compara- 
tive anatomy and physiology, affords us a con- 
vincing proof of the great advantage that may 
Ksolt from the cultivation of those sciences ; to 
which Nature^ uideed, sems, in this instance, 
cQtpressly to have invited us, by displaying to 
our view, in the organs of the circulation, an 
endless diversity of combinations, as if she had 
pnrposdy designed to elucidate their relations 
with the vital powers, and to assist our inves* 
ligations of the lawa of organized beings. 

VOL. II. u 


Chapter XI. 


§ 1 . Respiration in General. 

The action of atmospheric air is equally neces- 
saiy for the maintenance of animal, as of v^e- 
table life ; and as the ascending sap of the one 
cannot be perfected miless exposed to the che- 
mical agency of air in the leaves^ in like manner 
the blood of animals requires the perpetual reno* 
ration of its vital properties by the purifying in* 
fluence of respiration. The great importance of 
this function is evinced by the constant provision 
which has be^n made by Nature, in every class 
of animals, for bringing each portion of their 
nutritive juices, in its turn, into contact with ain 
Even the circulation of these juices is an object 
of inferior importance, compared with their 
aeration ; for we find that insects, which have 
but an imperfect and partial circulation of their 
blood, still require the free introduction of air 
into every part of their system. The necessity 
for air is more urgent than the demand for food ; 
many animals being capable of subsisting for sl 


oonijiderable time without nourishment, but all 
st>eedil>^p6rishiDg when deprived of air. The 
mfluence of thid element is requisite as well for 
the production and developement, as for the con-* 
tinuance of organized beings in a living state. 
No vegetable seed will germinate, nor will any 
^SSf ev^n of the smallest insect, give birth to a 
larva, if kept in a perfect vacuum. Experiments 
on thiia subject have been varied and multiplied 
without end by Spallanzani, who found that 
insects under an air pump, confined in rarified 
air, in general lived for shorter periods in pro- 
portion to the degree to which the exhaustion of 
air had been carried. Those species of infu- 
soria, which are most tenacious of life, lived in 
very rarefied air for above a month : others 
perished in fourteen, eleven, or eight days ; and 
some in two days only. In this imperfect 
vacuum, they were seen still to continue their 
accustomed evolutions, whedtng in circles, dart- 
ing to the surface, or diving to the bottom of the 
fluid, and {producing vortices by the rapid vibra- 
tion of their cilia, to catch the floating particlei^ 
which sierve as their food : in course of time, 
however, they invariably gave indications of un- 
easiness; their movements became languid, a 
gjeneral relaxation ensued, and they at length 
expired. But when the vacuum was rendered 
pcarfect, none of the infusions of animal or vege- 
table substances, which, under ordinary circuni- 


stances, soon swann with millions of these micros 
scopic beings, ever exhibited a sihglei^ animal^ 
cule ; although these soon made their appearance 
in great numbers, if the smallest quantity of aiv 
was admitted into the receiver. 

Animals which inhabit the waters, and remsan 
constantly under its surface, such as fishes^ and 
the greater number of mollusca, are necessarily 
precluded from receiving the direct action of 
atmospheric air in its gaseous state. But as all 
water exposed to die air soon absorbs it in large 
quantities, it becomes the medium by which that 
agent is applied to the respiratory organs of 
aquatic animals ; and the oxygen it contains miqr 
thus act upon the blood with considerable eflfect ; 
though not, p^haps, to the same extent as when 
directly applied in a gaseous state. The air 
which is present in water is, accordingly, as 
necessary to these animals as the air of the 
atmosphere is to those which live on laud : hence 
in our inquiries into the respiration of aqnatic 
animals, it will be sufficient to trace the means 
by which the surrounding water is allowed to 
have access to the organs appropriated to this 
function ; and in speaking of the action of the 
water upon them, it will always be understood 
that I refer to the action of the atmospheric air 
which that water contains. 

Respiration, in its different modes, may be 
distinguished, according to the nature of the 


medium which is breathed, into aquatic or atmos* 
ykaric i and in the fomer case, it is either ctito- 
netmSf or branchial^ according as the respiratory 
fNCgaiis are external or internal. Atmosphmc 
respiration, again, is either tracheal, or pulmo-' 
nary, according as the air is received by a 
system of air tubes, or trachea, or into pulmo- 
naiy caykies, or lungs. 

^i. Aquatic lieqnratian. 

Zoophytes appear in general to be unprovided 
with any distinct channels for conveying aerated 
water into the interior of their bodies, so that it 
may act in succession on the nutritive juices, 
and after perlbmuDg this office, may be expelled, 
and exchanged for a fresh supply. It has ac- 
eordingly been conjectured, oa the presumption 
diat this function is equally necessary to them 
as it is to all other animals, that the vivifying 
influence of the surrounding element is exerted 
ducDugh the medium of the sur&ce of the body. 
Thus it is very possible that in Poiy pi, while the 
interior surface of the sac digests the ibod, its 
extasmal surfetce may perfonn the office of res* 
piration : and no other mode of accompUshmg 
this function has been distinctly traced in the 
Acakpha. Medusee, indeed, appear to have a 
fiirther object than mere progression in the 


alternate ezpansioiifl and contractions of the 
floating edges of their hemispherical bodies ; for 
these movements are performed with great regn- 
larity under all circumstances of rest or motion ; 
and they continue even when the animal is taken 
out of the water and laid on the ground, as long 
as it retains its vitality. The specific name of 
the Medusa pulnu>'^ (the Pulmane Marino of the 
Italians), is derived from the supposed resem- 
blance of these movements to those of the lungs 
of breathing animals. The large cavities ad-- 
jacent to the stomach, and which have been 
already pointed out in Fig. 249 and 252,t have 
been conjeetured to be respiratory organs, chiefly, 
I believe, because they are not known to serve 
any other purpose. 

llie Entozoa, in like manner, present no ap- 
pearance of internal respiratory organs ; ao that 
they probably receive the influence of oxygen 
only through the m^um of the juices of the 
animals on ^hidi they subsist. PlanaruBy which 
have a more independent existence, though en^ 
dowed with a system of circulating vessels, have 
no internal respiratory organs; and whatever 
respiration they perform must be whdUy cuta- 
neous. Such is also. the condition of several of 

^ See the delineation of this animal in Fig. 135, toL i. p. 276. 
t Pages 86 wi 87 of thni Tolume. 


the dimpler kinds of AhneHda; but in those: 
which are more highly organized, an apparatus 
is provided for respiration, which is wholly ex- 
ternal to the body, and appears as an appendage 
to it, consistiBg generally of tufts of projecting 
fibres, branching like a plume of feathers, and^ 
floating in ihe surrounding fluid. The JLum- 
brictis marinvSj or lob-worm,* for example/ has 
two rows of branchial oigans of this description, 
one on each side of the body ; each row being 
composed of from fourteen to sixteen tufts. In 
the more stationary Annelida, which inhabit 
oalcaieous ttib^, as the Serpula and the Teredo, 
these arborescent tofts are protected by a sheath, 
which envelopes their roots; and they are j^aeed 
on the head, as being the only part which comes 
in contact with the water. 

Most of the smaller Crustacea have branchiee 
in the form of ^ feathery tufts, attached to the 
paddles near the tail, and kept in incessant 
vibmtory motion, which gives an appearance of 
great liveliness to the animal, and is more 
especially striking in the microscopic species. 
The variety of shapes which these organs assume 
in different tribes is too great to allow o£ any 
specific description of them in this place : but 

* Arenicola piscatoruxn (Lam.)- See a delineation of this 
marine worm in Fig. 136, vol. i. p. 276: 


aimdit these Taiieties it is sufficiently apilarant 
tliat their construction has been, in all cases, d^ 
Mgned to obtain a considerable extent of surface 
over which the minute subdivisions of the blood- 
vessels might be spread, in order to expose them 
fully to the aetimi of aerated water. 

The Mdlusca, also, present great diversity in 
the forms of their respiratory organs^ although 
they are all, with but a few exceptions, adapted' 
to aquatic respiration. In many of the tribes 
which have no shelly as the Thetis^ the Daris^ and 
the Tritania^ there are arborescent gills pvqecting 
from different parts of the body, and floating in 
the water. • In the Lepasy or barnacle, a curious 
family, constituting a connteting link between 
mdluscouB and articulated animals, these organs 
are attached to the bases of the cirrhi, or jointed 
tentacula, which are kept in omstant motion, 
in order to obtain the full action of the water on 
the blood-vessels they contain. 

We are next to consider the extensive series 
of aquatic animals in which respiration is carried 
on by organs situated in the interior of the body. 
The first example of internal aquatic respiration 
occurs in the Holothuriaj where there is an 
OTgan composed of ramified tubes, situated in a 
cavity communicating with the intestine, and 
having an external opening for the admission of 
the aerated water, which is brought to act on a 


imscnlar net-work, containinff the nutritiTe mices 
rf ..he «umd. »d .ppJoUy perfo^Lg . 
partial ciicnlatioii of those juices. A still more 
complicated syst^n of respiratory channels 
ocdUB, both in the Echinus and Asterias^ where 
they open by separate, but very minute on&ces, 
distinct from the laiger apertures through which 
tlie feet protrude; and the water admitted 
thdrcmgh these tubes is allowed to permeate the 
gp^seral cavity of the body, and is thus brought 
into contact with all the wgans. 

The animals composing the family oi AsciduB 
have a large respiratory cavity, receiving the 
water from without, and having its sides lined 
with a membrane, which is thrown into a great 
mmber of folds; thus considerably extending 
the surface on which the water is designed to 
act The entrance into the oesophagus, or true 
mouth, is situated at the bottom of this cavity ; 
that is, at the part most remote from the ex- 
ternal orifice ; so that all the food has to pass 
through the respiratory cavity, before it can 
be swallowed, and received into the stomach. 

In several of the Annelida^ also, we find in- 
ternal organs of respiration. The Lumbricus'ter- 
restrisj or common earth-worm, has a single row 
of apertures, about 120 in number, placed along 
the back, and opening between the segments of 
the body: they each lead into a respiratory 


veside, sitaated between the integoment end the 
intestine.* The Ije9ch has sixteen minute oii* 
fiees of this kind on each side of the body, open- 
ing internally into the same number of oval cells, 
which are respiratory caritieB ; the water passing 
both in and out by the same onfices.f 

The Aphrodita aculeata bsA thirty-two CNrifices 
on each side, placed in rows, opening into one 
large respiratory sac, which is situated immedi^ 
ately under the muscles of the back, but sepa* 
rated by a membrane from the abdominal cavity. 
Prc^cting' into this sac, are seen seyeral mem- 
branous vesicles, fifteen* in number on each 
side, which have no external opening, but which 
receive, on the inner part, the ends of certain 
tubes, or cseca, sent off from the intestinal canal ; 
so that the nutriment is aerated almost as soon 
as it is prepared by the digestive organs.^ 

In all the higher classes of aquatic animals, 
where the circulation is carried on by means 
of a muscular heart, and wh«e the whole of 
the^ blood is subjected, during its circuit, to the 

* A minute description of these orgwi is given by Morren, in 

pages 53 and 148 of his work already quoted. 

t The blood after being aerated in these cells^ is conveyed 
into the large lateral vessels, by means of canals^Which pass 
tnmsversdy, forming loops, situated between the csBca of the 
Blomagh. These loops are studded with an immense number of 
small rounded bodies of a glandular appearance, resembling those 
which convey the vensB cavee of the cephalopoda. 

t Home, Philos. Trans, for 1815, p. 259. 


action of the aerated water, the immediate organs 
of respiration consist of long, narrow filaments 
in the form of a fringe, and the blood-vessds 
belonging to the respiratory system are exten- 
siTely distributed oyer the whole surfiice of .these 
filaments. Organs of this description are deno-. 
minated BranchuBy or Grills; and the arteries 
which bring the blood to them are. called the 
hroM/ehial avieties; the veins,. which convey it 
back, being, of course, the branchial veins. 

The larger Cfnutaeea have their branchiae 
situated on the under Mde of the body, not only 
in order to obtain protection from the carapace, 
which is folded over them, but also for the sake 
of being attached to the haunches of the feet* 
jiiws, and thoracic fe^ and thus participating in. 
the movements of those oi^ans. They may be 
seen in the . Lobster, or in tha Crab, by raising 
the lower edge of the carapace. The form of 
each branchial lamina is shown at g, in Fig, 
354*: they consist of assemblages of many 
thousands of minute filaments, proceeding from 
their respective stems, like the fibres of a feather ; 
and each group having a triangular, or pyra- 
midal figure; The number of these pyramidal 
bodies varies in the difierent genera ; thus the 
Lobster has twenty-two, disposed in vows on 
each side of the body; but in the Crab, there 


* Page 269 of this volume. 


are only serea on each side. To these organs 
are attached short and flat paddles, which are 
moved by appropriate muscles, and are kept in 
incessant motion, producing strong currents of 
water, evidently for the purpose of obtaining the 
foil action of the element on every portion of the 
surface of the branchiae. 

In the greater number of Mollusca, these im-* 
portant organs, although external with respect 
to the viscem, are within the shell, and are 
generally situated near its outer mai^in. They 
are composed of parallel filaments, arranged like 
the teeth of a fine comb ; and ah opening exists 
in the mouth for admitting the water which is 
to act upon them.* In the Gasteropoda^ or 
inhabitants of univalve shells, this opening is 
usually wide. In the Aeephala^ or bivalve mol* 
lusca, the gills are spread out, in the fwm of 
laminae, round the margin of the sheU, as is 
exemplified in the oyster, where it is commonly: 
known by the name of heard. The aerated 

* These filaments appear, in many instances, to have the 
power of producing currents of water in their Ticinity by the 
aotion of niinvte cilia, similar to those belonging to the tentacyln 
of many polypi, where the same phenomenon is observable. 
Thus if one of the branchial filaments of the fresh water muscle 
be cut across, the detached portion will be 'seen to advance iti 
the fluid by a spontaneous motion, like the teDtacoluBi of a 
polype, under the same circumstatices. Similar currents of 
water, according to the recent observations of Mr. Lister, and 
apparently determined by the same mechanism of vibratory cilia, 
take place in the branchial sac of AscidtflB. 


water is admitted through a fissure in the 
mouth, and when it has performed its office 
in respiration, is usually expelled by a sepa- 
rate opening. The part of the mouth through 
which the water is admitted to the branchiae is 
sometimes prolonged, forming a tube, open at 
the extremity, and at all times allowing free 
ingress and egress to the water, even when the 
animal has withdrawn its body whoUy within 
itft sheU. Sometimes one, and sometimes two 
tubes of this kind are met with ; and they are 
often protected by a tubular portion of shell, as 
is seen in the Murex^ Buccinum^ and Strambus ; 
in other instances the situation of the tube is 
only marked by a deep notch in the edge of the 
shell. In those moUusca which burrow in the 
sand, this tube can be extended to a considerable 
length, so as to Veach the water, which is alter- 
nately sucked in and ejected by the muscular 
acticm of the mouth. In those Acephala which 
are improTided with any tube of this kind, the 
mechanism of respiration consists simply in the 
opening and shutting of the shell. By watch- 
ing them attentively we may perceive that the 
surrounding water is moved in an eddy by these 
actions, and that the current is kept up without 
interruption. All the Sepise have their gills en- 
closed in two lateral cavities, which communi- 
cate with a funnel-shaped opening in the middle 
of the neck, alternately receiving and expelling 


the water by the muscular action of its eides. 
The fonuB assumed by the respiratoty oi:gan» in 
this class are almost infinitely diversified, while 
the geaaeral design of their arrangement is still 
the aiatne. 

As we rise in the scale of animals, tbe reapira* 
tory function assumes a higher importance. In 
Fishes the gills form large organs, and the c<«- 
tinuance of their action is more essential to life 
than it appears to be in any of the inferior 
classes: they are situated, as is well known, on 
each aide of the tliroat in the immediate Ticinity 
of the heart. Their usual form is shown at o o. 

Fig. 366, where they are represented on one side 
only, but in their relative sitoations with respect 
to the auricle (d), and Tentricle (e), of the heart; 
the bulbus arteriosus (b), and the branchial ar- 


tery (f). They have thei same fringed structure 
as in the moUusca, the fibres being, set close to 
eltch other, like the barbs of a feather, or the 
teeth of a fine comb, and being attached, on each 
side of the throat, in double rows, to the convex 
miargins of four cartili^noud or osseous arches, 
which are themselves connected with the jaws 
by the Ixme called the os hyoides. The mode of 
their artictdation is such as to allow each arch 
to have a small motion forwards, by which they 
are separated from one another ; and by moving 
backwards they are again brought together^ 
or c<^{>8ed. Each filament conitaids a slender 
plate of cartilage, giving it mechanical sup^ 
port, and enabling it to preserve its i&hape 
while moved by the streams of water which 
are perpetually rushing past. When their sur- 
faces are Still more minutely examined, they 
are found to be covered with innumerable 
minute processes, crowded together like the pile 
of velvet ; and on these are distributed myriads 
of blood-vessels, spread, like a delicate net- work, 
over every part of the surface. The whole 
extent of this surface exposed to the action of 
the aerated water, by these thickly set filaments, 
must be exceedingly great.* 

A large flap, termed the Operculum^ extends 
oter the whole oi^an, defending it from injury, 

* Dr. Monro computed that in the skate, the surface of the 
gills ifl, at the least, equal to the whole sur&ce of the human 


and leaving below a wide fissure for the escape 
of the water, which has performed its office in rai- 
piration^ For this purpose the water is taken ia 
by the mouth, and forced by the muscles of the 
throat through the apertures which lead to the 
branchial cavities : in this action the branclual 
arches are brought forwards and separated to a 
certain distance from each other ; and the rush 
of water through them unfolds and separates 
each of the thousand minute filaments of the 
branchiae, so that they all receive t^e full action 
of that fluid as it passes by them. Such appears 
to be the principal mechanical object of the act 
of respiration in this class of animals ; and it is 
an object that requires the co-operation of a 
liquid such as water, capable of acting by its 
impulsive momentum in expanding every part 
of the apparatus on which the blood vessels are 
distributed. When a fish is taken out of the 
water, this efiiect can no longer be produced ; in 
vain the animal reiterates its utmost efforts to 
raise the branchiae, and relieve the sense of 
suffocation it experiences in consequence of the 
general collapse of the filaments of those organs, 
which adhere together in a mass, and can no 
longer receive the vivifying influence of oxygen*. 

* It has been generally stated by physiologists, even of the 
highest authority, such as Cuvier, that the principal reason why 
fishes cannot maintain life, when surrounded by air instead of 
water, is that the branchiae become dry, and lose the power of 


Death is, in like manner, the con&equence of 
a ligature passed round the fish, and preventing 
the expansion of the branchiae and the motion of 
the opefTuIa. 

In all osseous fishes the opening under the 
operculum for the exit of the respired water, is 
a simple fissure; but in most of the cartilaginous 
tribes, there is no operculum, and the water 
escapes through a series of apertures in the 
side of the throat. Sharks have five oblong 
orifi<^e8 of this description, as may be seen in 

Fig. S67t. 

As the Lamprey employs its mouth more con- 
stantly than other fish in laying hold of its prey, 
and adhering to other bodies, the organs of res- 
piration are so constructed as to be independent 
of the mouth in receiving the water. There are 
seven external openings on each side (Fig. 368), 
leading into the same number of separate oval 
pouches, situated horizontally, and the inner 
membrane of which has the same structure 
as gills: these pouches are seen on a larger 

acting when thus deprived of their natural moisture : for it might 
iitherwise naturally be expected that the oxygen of atmospheric 
air would exert a more powerful action on the blood which cir- 
culates in the branchiee, than that of merely aerated water. 
The rectification of this error is due to Flourens, who pointed 
oat the true cause of suffocation, stated in the text, in a Memoir 
entitled *^ Experiences sur le M^chanisme de la Respiration des 
Poissons.** — Annales des Sciences Naturelles, xx, 5. 

t They are also visible in Fig. 293, (page 166), which is that 
of the Squalus pristiSj a species belonging to this tribe. 

VOL. 11. X 


scale than in the preceding figure, in Fig. 309. 
There is also an equal number of internal 
openings, seen in the lower part of this last 
figure, leading into a tube, the lower end of 
which is closed, and the upper terminates by 
a fringed edge in the oesophagus. Hie watet 
which is received by the seven lateral openings, 
enters at one side, and after it has acted upon the 
gills, passes round the projecting membranes. 
The greater part makes its exit by the same 
orifices ; but. a portion escapes into the middle 
tube, and thence passes, either into the oth^ 
cavities, or into the oesophagus*. 

In the Myxine^ which feeds upon the internal 
parts of its prey, and buries its head and part 
of its body in the flesh, the openings of the 
respiratory orgaDos are removed sufficiently &r 
from the head to admit of respiration going on 
while the animal is so employed ; and there are 
only two external openings, and six lateral 
pouches on each side, with tubes similar to those 
in the lamprey. 

The Perca scandefis (Daldorff)t, which is a 
fish inhabiting the seas of India, has a very 
remarkable structure adapting it to the main- 

* It was commonly supposed that the respired water is ejected 
through the nostril : but this is certainly a mistake, for the 
nostril has no communication with the mouth, as was pointed 
out by Sir £. Home. Phil. Trans, for 1815, p. 259. These 
organs have also been described by Bloch and Gcertner. 

t Anthias testudineus (Bloch) : AncAas (Cuv.) 


tmance of respiration, and consequently to the 
support of life for a considerable time when out 
of the water : and hen6e it is said occasionally 
to traydi on land to some distance from the 


coast*. The pharyngeal bones of this fish have 
a foliated and cellular structure, which ^ves 
them a capacity for retaining a sufficient quan- 
tity of water, not only to keep the gills moist, 
but also to enable them to perform their proper 
office ; while not a' particle of water is suffered 
to escape from them, by the opercula being 
accurately closed. 

The same faculty, resulting from a similar 
gtracture, is possessed by the Ophicephalns^ 
which is also met with in the lakes and rivers of 
India and China. Eels are enabled to carry on 
respiration when out of water, for a certain 
period, in consequence of the narrowness of the 
aperture for the exit of the water from the bran- 
chial cavity, which enables it to be closed, and 
the water to be retained in that cavity .f 

I have already stated that, in all aquatic ani- 
mals, the water which is breathed is merely the 
vehicle by which the air it contains is brought 
into contact with the organs of respiration. This 

* This peculiar faculty has been already alluded to in 
-Volume i, p. 433. 

t Dr. Hancock states that the Doras cottatu^^ {Silurus cos- 
tatus^ Linn.) or Hassar, in very dry seasons, is sometimes seen, 
in great numbers, making long marches over land, in search of 
crater. Edin. Phil. Journal, xx. 396. 


ur is constantly vitiated by the respiration of 
these animals, and requires to be renewed by 
the absorption of a fresh portion, which can 
only take place when the water fredy commu- 
nicates with the atmosphere: and if this renewal 
be by any means prevented, the water is no 
longer capable of sustaining life. Fishes are 
killed in a very few hours, if confined in a 
limited portion of water, which has no access 
to fresh air. When many fishes are enclosed in 
a narrow vessel, they all stni^le for the upper- 
most place, (where the atmospheric air is first 
absorbed,) like the unfortunate men imprisoned 
in the black-hole at Calcutta. When a small 
fish-pond is frozen over, the fishes soon perish, 
unless holes be broken in the ice, in order to 
admit air : they may be seen flocking- towards 
these holes, in order to receive the benefit of 
the fresh air as it is absorbed by the water; 
and so great is their eagerness on these occa* 
sions, that they often allow themselves to be 
caught by the hand. Water, from which all 
ttir has been extracted, either by the air-pump, 
or by boiling, is to fishes what a vacuum is to 
a breathing terrestrial animal. Humboldt and 
Provencal made a series of experiments on the 
quantities of air which fishes require for their res- 
piration. They found that river-water generally 
contains about one 36th of its bulk of air, of 
which quantity one-third consists of oxygen, 


being about one per cent, of the whole volume. 
A tench is able to breathe when the quantity of 
oxygen is reduced to the 5000th part of the bulk 
of the water, but soon becomes exceedingly 
feeble by the privation of this necessary ele- 
ment. The fact, however, shows the admirable 
perfection of the organs of this fish, which can 
extract so minute a quantity it air from water 
to ^hich that air adheres with great tenacity*. 

* The swimmiDg bladder of fishes is regarded by many of the 
German naturalists as having some relations with the respiratory 
fnnction, and as being the rudiment of the pulmonary cavity 
of land animals ; the passage of communication with the oeso*- 
phagus being conceived to repre^nt the trachea. The air con- 
tained in the swimming bladder of fishes has been examined by 
many chemists, but although it is generally found to be a mixture 
of oxygen and nitrogen, the proportion in which these gases exist 
is ofasevyed to vary considerably. Biot concluded from his expe^ 
riments^ that in the air-bladder of fishes inhabiting the greatest 
depths of the ocean, the quantity of oxygen is greater, while in 
those of fishes which cokne often to the surface, the nitrogen is 
mpre abundant ; and De la Roche came to the same conclusion 
frpm his researches on the fishes of the Mediterranean, From the 
experiments of Humboldt and Provenqal, on the other hand, we 
may conclude, that the quality of the air contained in the air- 
bladder is i)ut remotely connected with respiration. (Memoires 
de la Society d'Arcueil, ii, 359.) 

According to Ehrmann, the CobitiSf or Loche, occasionally 
swallows air, which is decomposed in the alimentary canal, and 
effects a change in the blood-vessels, with which it is brought 
into contact, exactly similar to that which occurs in ordinary 
respiraUon. It is also believed that in all fishes a partial aeration 
of the blood is the result of a similar action, taking place at the 
surface of the body under the scales of the integuments. Cuvier, 
•ur les Poissons, I, 383. 


§ 3. Atmospheric Respiration. 

The next series of structures which are to come 
under our review^ comprehends all those adapted 
to the respirattu of atmospheric air in its 
gaseous form ; and their physiology is no less 
diversified than that of the organs by which 
water is respired. 

Air may be respired by tracheaj or by pul- 
monary cavities ; the first mode is exemplified in 
insects ; the second is that adopted in the larger 
terrestrial animals. 

The greater part of the blood of insects being 
difihsed by transudation through every internal 
organ of their bodies, and a small portion only 
being enclosed in vessels, and circulating in them» 
the salutary influence of the air could not have 
been generally extended to that fluid by any of 
the ordinary modes of respiration, where the 
function is carried on in an organ of limited extent. 
As the blood could not be brought to the air, it 
became necessary, therefore, that the air should 
be brought to the blood. For this purpose there 
has been provided, in all insects, a system of 
continuous and ramified vessels, called trache^e^ 
distributing air to every part of the body. The 
external orifices, firom which these air tubes 


commence, are called spiracles, or stigmata, taid 
are ueaally eituated in rows oa each side of 

the body, as is shown in Fig. 370, which repre- 
sents the lower abdominal surface of the Dytit- 
cua marginalis. They are seen Tery distinctly 
ia the caterpillar, which has generally ten on 
each side, corresponding to the number of abdo- 
minal segments. In many insects we find them 
guarded by bristles, or tufts of hair, and some- 
times by valves, placed at the orifice, to prevent 
the raitrance of extraneous bodies. The spira- 
cles are opened and closed by muscles provided 
for that purpose. Fig. 371 is a magnified view 
of spiracles of this description, from the Ceram- 
byx keros. (Fab.) They are the beginnings of 
short tubes, which open into large trunks (as 
^own in Fig. 372), extending longitudinally 


OQ each side, and sending off radiating branches 
£rom the parts which are. opposite to the spi- 
racles; and these branches are further sufodi- 
videdy in the same manner as the arteries of the 
larger animals, so that their minute ramifications 
pervade every organ in the body. This ramified 
distribution has frequently occasioned their 
being mistaken for blood vessels^ In the wings 
of insects the nervures, which have the appear- 
ance of veins, are only large air-tubes. Jurine 
asserts that it is by forcing air into these tubes 
that the insect is enabled suddenly to expand 
the wings in preparing them for flight, giving 
them by this means greater buoyancy as well as 

The trachese are kept continually pervious by 
a curious mechanism ; they are formed of three 
coats, the external and internal of which are 
nvembranous ; but the middle coat is constructed 
of an elastic thread coiled into a helix, or cylin- 
drical spiral (as seen in Fig. 372) ; and the 
elasticity of this thread keeps the tube constantiy 
in a state of expansion, and therefore full of air. 
When examined under water, the tracheae have a 
shining silvery appearance, from the air they 
contain. This structure has a remarkable ana- 
logy with that of the air vessels of plants, which 
also bear the name of trachese ; and in both 
similar variations are observed in the contexture 


of the elastic membrane by whicb they are kept 

' The tracheee, in.many parts of their course, pre- 
sent remarkable dilatations, which apparently 
serve as reservoirs of air ; they are very conspi- 
.cuous in the Dytiscus marginalise which resides 
principally in water; but they also exist in 
many insects, as the Mehlontha and the Ceram- 
hyx^ which live wholly in the air.t Those of 
the Scolia Aortorum (Fab.) are delineated in 
JFig. 373, considerably magnified. 

If an insect be immersed in water, air will be 
seen escaping in minute bubbles at each spi- 
racle ; and in proportion as the water enters into 
the tubes, the sensibility is destroyed. If all the 
spirficles be closed by oil, or any other unctuous 
substance, the insect immediately dies of sui- 
focation ; but if some of them be left open, 
respiration is kept up to a considerable extent, 
from the numerous communications which exist 
among the air vessels. Insects soon perish when 
placed in the receiver of an air-pump, and the 

♦ According to the observation of Dr. Kidd these vessels are 
often annular in insects, as is also the case with those of plants. 
He considers the longitudinal trachece as connecting channels, 
by which the insect is enabled to direct the air to particular parts 
for occasional purposes. Phil. Trans, for 1825, p. 234. 

t Leon Dufour, Annales des Sciences Naturelles ; viii. 26. 

: Ibid p. 232. 


air exhausted ; but they are generally moie te- 
nacious of life under these circumstances than 
the larger animals^ and often, aftw being appa- 
rently dead, revive on the readmission of air. 

Aquatic insects have trache®, like those living 
in air, and are frequ^itly provided with tubes, 
which are of sufficient length to reach the sur- 
face of the water, where they absorb air for res- 
piration. In a few tribes a complicated mode 
of respiration is practised; aerated water is 
taken into the body, and introduced into cavities, 
when the air is extracted horn it, and trans- 
mitted by the ordinary tracheae to the different 
parts of the system.* 

Such, then, is the extensive apparatus for 
aeration in animals, which have either no circu- 
lation of their nutritious juices, or a very im- 
perfect one ; but no sooner do we arrive at the 
examination of animals possessing an enlarged 
system of blood vessels, than we find nature 
abandoning the systan of tracheae, and employ^ 
ing more simple means of effecting the aeration 

* Mr. Dutrochet conceives that the principle on which this 
operation is conducted is the same with that by which gases are 
reciprocally transmitted through moistened membranes ; as in 
the experiments of Humboldt and Gay Lussac, who, on enclosmg 
mixtures of oxygen, nitrogen, and carbonic acid gases, in any 
proportion, in a membranous bladder, which, was then immersed 
in aerated water, found that there is a reciprocal transit of 
the gases ; until at length pure atmospheric air remains in the 
cavity of the bladder. 


of the blood. Advantage is taken of the facility 
afforded by the blood-Tessels of transmitting the 
Mood to particular organs, where it may con- 
voiiently receive the influence of the air. Thus 
Scorpions are provided, on each side of the 
thorax, with four pulmonary cavities, seen at l, 
on the left side of Fig. 374, into each of which 

air is admitted by a separate external opening, 
A, B, is the dorsal vessel, which is connected with 
the pulmonary cavities by means of two sets of 


muscles, the one set (xM, m) being longer than the 
other (m, m, m). The branchial arteries (v) are 
seen ramifying over the inner surface of the pul- 
monary cavities (r) on the right side, whence 
the blood is conveyed by a correq[K)nding set of 
branchial veins to the dorsal vessel ; and other 
vessels, which are ordinary veins, are seen at o, 
proceeding from the abdominal cavity to join the 
dorsal vessel. The membrane which lines the 
pulmonary cavities is curiously plaited, presett- 
ing the appearance of the teeth of a comb, and 
partaking of the structure of gills ; and on this 
account these organs are termed by Latreille 
pneumo-branchite . Organs of a similar descrip- 
tion exist in Spiders, some species having eight, 
others four, and some only two: but there is 
one entire order of Arachnida which respire by 
means of tracheae, and in these the circulation 
is as imperfect as it is in insects. 

It may here be remarked that an essential dif- 
ference exists in the structure of the respiratory 
organs, according to the nature of the medium 
which is to act upon them : for in aquatic res- 
piration the air contained in water is made to 
act on the blood circulating in vessels which 
ramify on the external surface of the filaments 
of the gills ; while in atmospheric respiration 
the air in its gaseous state is always received 
into cavities, on the internal surface of which the 
blood-vessels, intended to receive its influence. 


are ditrtributed. It is not difficult to assign the 
final cause of this change of plan ; for in each 
case the structure is accommodated to the me- 
chanical properties of the medium respired. A 
liquid, bdng inelastic and ponderous, is adapted; 
by its momentum alone, to separate and sur- 
round the loose floating filamdits composing the 
branchiae ; but a light gaseous fluid, like air, is 
on the contrary, better fitted to expand dilatable 
cavities into which it may be introduced. 

Occasionally', however, it is found that organs 
constructed like branchiae, and usually perform* 
ing aquatic respiration, can be adapted to respire 
air. This is the case with some species of Crus- 
tacea, of the order Decapoda^ such as the Crab, 
which, by means of a peculiar apparatus, dis- 
covered by Audouin and Milne Edwards, retain 
a quantity of water in the branchial cavity so as 
to enable them to live a very long time out of the 
water. It is only in their mature state of de- 
velopement, however, that they are qualified for 
this amphibious existence, for at an early period 
of growth they can live only in water. ' ' 

There, is an entire order of Gastbropodous 
MoUusca which breathe atmospheric air by 
meanft of pulmonary cavities. This is the case 
with the LinuuCf or slug, and also with the 
Helix J or snail, the Testaeellaj the Clausilia; 
and many others, which, though partial to moist 
situations, are, from the conformation of their 


leqMntoiyoigaii&yeaBentiaUylandiHm^^ The 
air b received by a round sqiotiire Bear the 
head, guarded by a qphincter muacle, which is 
seen to dilate or contract as occaooii may le* 
quire, bnt which is sometimes completdy coq- 
cealed from view by the month feldiiig over it. 
The cavity, to which this opeoing leads^ is lined 
with a membrane delicately Added, and over- 
spread with a beantifal net-woric of pobnenary 
vessels. Other moUosca of the same order, 
which are more aquatic in their habits, have 
yet a similar structure, and are obliged at in- 
tervals to come to the sur&ce of the water in 
order to breathe atmospheric air: this is the 
case with the Onchidium^ the Pkmariis, the 
JLymrueaj &c. 

The structure of the pulmonary oigans be- 
comes gradually more refined and com|dicated 
as we ascend to the higher classes of animals. 
In all v^rtebrated terrestrial animals they are 
called lungSy and consist of an assemblage of 
vesicles, into which the air is admitted by a 
tube, called the trachea^ or wind-pipe, extending 
downwards fram the back of the mouth, parallel 
to the oesophagus. Great care is tak^i to guard 
the banning of this passage from the intrusion 
of any solid or liquid that may be swallowed. A 
cartilaginous valve, termed the epigtottisy is 
generally provided for this purpose, which is 
made to descend by the action of the same 


muscles that perform deglutition, and which 
then closes accurately the entrance into the air- 
tube. It is an exceedin^y beautiful contriy- 
ance, both as to the simplicity of the 
and the accuracy with whicdi it accompli 
the purpose of its formation. At the upper 
part of the chest the trachea divides into two 
branches, called the bronchia^ passing to the 
lungs on either side. Both the wind*pipe and 
the bronchia are preyented from closing by the 
interpositicm of a series of firm cartilaginous 
ringlets, interposed between their inner and 
outer coats, and placed at small and equal dis- 
tances from one another. The natural elasticity 
of these ringlets tends to keep the sides of the 
tube stretched, and causes it to remain open : 
it is a structure very analogous to that of the 
trachea of insects, or of the vessels of the same 
name in plants. 

The lungs of Reptiles consist of large sacs, 
into the cavity of which the bronchia, proceed- 
ing from the bifurcation of the trachea, open at 
once, and without fiirther subdivision. Cells are 
formed within the sides of this great cavity, by 
fine membranous partitions, as thin and delicate 
as soap bubbles. The lungs of serpents have 
scarcdly any of these partitions, but consist of 
one simple pulmonary sac, situated on the right 
side, having the slender elongated form of all 
the other viscera, and extending nearly the 


whole length of the body. The long on die left 
side is in general scarcely discernible, being 
very imperfectly developed. In the chamelion 
the lungs have numerous processes which pro* 
ject from them ' like cesca. In the Sauria, the 
lungs are more confined to the thoracic region, 
and are more completely cellular. 

The mechanism, by which, in these animals, 
the air is forced into the lungs, is exceedii^ly 
peculiar, and was for a long time a subject of 
controversy. If we take a frog as an example, 
and watch' its' respiration, we cannot readily dis- 
cover that it breathes at all, for it never opens 
its mouth to receive air, and there is no motion 
of the sides to indicate that it respires ; and 
yet, on any sudden alarm, we see the animal 
blowing itself up, as if by some internal power, 
though its mouth all the while continues to be 
closed. We may perceive, however, that its 
throat is in frequent motion, as if the frog were 
economising its mouthful of air, and transferring 
it backwards and forwards between its mouth 
and lungs ; but if we direct our attention to the 
nostrils, we may observe in them a twiriing 
motion, at each movement of the jaws; for it is, 
in £aict, through the nostrils that the frog receives 
all the air which it breathes. The jaws are 
never opened but for eating, and the sides of 
the mouth form a sort of bellows, of which the 
nostrils are the inlets; and by their alternate 


cimtraction and relaxation the air is swallowed, 
and forced into the trachea, so as to inflate the 
Inngs. If the mouth of a frog be forcibly kept 
open, it can no longer breathe, because it is 
deprived of the power of swallowing the air 
required for that function ; and if its nostrils be 
closed, it is, in like manner, suffocated. The 
respiration of most of the Reptile tribes is per- 
formed in a similar manner ; and they may be 
said rather to swallow the air they breathe, than 
to draw it in by any expansive action of the 
parts which surround the cavity of lungs; for 
even the ribs of serpents contribute but little, by 
their motion, to this effect, being chiefly useful 
as organs of progressive motion. 

The Chelonia have lungs of great extent, 
passing backwards under the carapace, and 
reaching to the posterior part of the abdomen. 
Turtles, which are aquatic, derive great advan- 
ts^es from this structure, which enables them 
to give buoyancy to thdr body, (aicumbered as 
it is with a heavy shell,) by introducing into it a 
large volume of air ; so that the lungs, in fact, 
serve the purposes of a laige swimming bladder. 
That this use was contemplated in their struc- 
ture is evident from the volume of air received 
into the lungs being much greater than is re- 
quired for the sole purpose of respiration. The 
section of the lungs o£ the turtle (Fig. 375), 



shows their interior stnicture, compoeed of largo 
cells, into which the trachea (t) opens. 

Few subjects in animal physi<^ogy are more 
deserving the attention of those whose object it 
to trace the operatiMis of nature in the pn^^res- 
sive derelopement of the organs, than the 
changes which occur in the evolution of the 
tadpole irom the time it leaves the egg till it has 
attained the form of the perfect frog. We have 
already had occasion to notice sevend of these 
transformations in the oi^ns of the mechanical 
functions, and also in those of digestion and 
circulation : but the most remarkable of all are 
the changes occurring in the respiratory appa- 
ratus, corresponding with the opposite nature of 
the elements which' the same animal is destined 
to inhabit in the different stages of its exist^ice. 


No less than three sets of oigans are provided 
for respiration; the two first being branchiae, 
adapted to the fish*like condition of the tadpole, 
and the last being pulmonary cavities, for re- 
ceiving air, to be employed when the animal 
exchanges its aquatic for its terrestrial life. It 
is exceedingly interesting to oliserve that this 
animal at first breathes by gills, which project 
in an arborescent form from the sides of the 
neck, and float in the water; but that these 
structures are merely temporary, being provided 
only to meet (he immediate exigency of the 
occasion, and being raised at a period when 
none of the internal organs are as yet perfected. 
As soon as another set of gills, situated inter* 
nally, can be constructed, and are ready to 
admit the circulating blood, the external gills 
are superseded in their office ; they now shrivel, 
and are removed, and the tadpole performs its 
respiration by means of branchiae, formed on the 
model of those of fishes, and acting by a similar 
mechanism* By the time that the system has 
imdergone the changes necessary for its conver- 
^on into the frog, a new apparatus has become 
evolved for the respiration of air. These are the 
lungs, which gradually coming into play, direct 
the current of blood from the branchiae, and take 
upon themselves the whole of the office of res- 
piration. The branchiae, in their turn, become 
useless, are soon obliterated, and leave no other 


trace of their former existence than the originial 
division of the arterial tranks, which had sup- 
plied them with blood directly from the hearty 
but which, now uniting in the back, form die 
descending aorta.^ 

There is a small family, called the Perenni- 
branchiae belonging to this class, which, instead 
of undergoing all the changes I have been des- 
cribing, present, during their whole lives^ a grelit 
similitude to the first stage of the tadpole. This 
is the case with the Axolotl^ the Proteus angui- 
nus^ the Siren laceriinay and the Menobranehns 
lateralis^ which permanently retain their external 
gills, while at the same time they possess imper- 
fectly developed lungs. It would therefore seem 
as if, in these animals, the progress of develope- 
ment had been arrested at an early stage, so that 
their adult state corresponds to the larva condi- 
tion of the frog.f 

In all warm blooded animals respiration be* 
comes a Amotion of much greater importance, 

• See Fig. 357, p. 274. 

t Geoffroy St. Hilaife thinks there isgronnd for believing that 
Crocodiles and Turtles possess, in addition to the ordinary pul- 
monary respiration, a partial aquatic abdominal respiration, 
efTected by means of the two channels of communication which 
have been found to exist between the cavity of the abdomen and 
the external surface of the body : and also that some anakgy 
may be traced between this aquatic respiration in reptiles^ by 
these peritoneal canals^ and the supposed function of the swim- 
ming bladder of fishes, in subserviency to a species of aerial res- 


the continuance of life being essentially depen- 
dent on its vigorous and unceasing exercise. 
The whole class of Mammalia have lungs of an 
iuceedingly deyeloped structure, ccMtnposed of 
an immense number of minute cells, crowded 
together as closely as possible, and presenting a 
vast extent of internal surface. The thorax, or 
cavity in which the lungs, together with the 
heart and its great blood-vessels, are inclosed, 
hw somewhat the shape of a cone ; and its sides 
are defended from compression by the arches of 
the ribs, which extend from the spine to the 
sternum, or breast-bone, and produce mechani- 
cal support on the same principle that a cask is 
fljtrengthened by being girt with hoops, which, 
though composed of comparatively weak mate- 
rials, are yet capable, from their circular shape, 
of presenting great resistance to any compress- 
ing force. ' 

While Nature has thus guarded the chest, with 
such peculiar solicitude, against the efforts of 
any external force, tending to diminish its capa- 
city, she has made ample provision for enlarging 
or c(mtracting its diameter in the act of respira- 
tion. First, at the lower part, or that which 
corresponds to the basis of the cone, the only 
side, indeed, which is not defended by bone, 
there is extended a thin expansion, partly mus- 
cular, and partly tendinous, forming a complete 
partition, and closing the cavity of the chest on 


the side next to the abdomen. This muscle is 
called the Diaphragm : it is perforated, close to 
its origin from the spine, by four tubes, namely, 
the oesophagus, the aorta,' the vena cava, and the 
thoracic duct. Its sur&ce is not flat, but convex 
above, or towafds the chest; and the direction of 
its fibres is such that when they contract they 
bring down the middle part, which is tendinous, 
and render it more flat than before, (the passage 
of the four tabes already m^itioned, not int«^ 
fering with this action,) and thus the cavity of 
the thorax may be considerably enlaiged^ It is 
obvious that if, upon the descent of the *dia* 
phragm, the lungs were to remain in their 6ri<^ 
ginal situation, an empty space would be left 
betweai them and the diaphragm. But ne 
vacuum can take place in the body; the air 
cells of the lungs must always contain, even in 
their most compressed state, a certain quantity 
of air : aind this air will tend, by its elasticity, to 
expand the cells; the lungs will consequently 
be dilated, and will continue to fill the chest ; ffiid 
the external air will rush in through the trachea 
in order to restore the equilibrium. This action 
is termed in^ration. The air is again thrown 
out when the diaphragm is relaxed, and pudied 
upwards, by the action of the large muscles of 
the trunk; the elasticity of the sides of the 
chest, concurring also in the same efiect; and 
thus expiration is accomplished. 


. The muscles nrhich moTe tke^ribe coiist>ure 
also to. produce ^latationa and contractions of 
the cavity of the chest. Each rib is capable of 
a small degree of motion on that extremity by 
which it is attached to the spine ; and this mo- 
tion, assuming the chest to be in the erect posi- 
tion, as in man, is chiefly upwards and down- 
wards. But, since the inclination of the ribs is 
such that their lower edges form acute angles 
with the spine, they bend downwards as they 
proceed towards the breast ; and the uppermost 
rib being a fixed point, the action of the inter* 
costal muscles, which produces an approximation 
of the ribs, tends to raise them, and to bring them 
more at right angles with the spine ; the sternum 
also, to which the other extremities of the ribs 
are articulated, is elevated by this motion, and 
consequently removed to a greater distance 
from the spine; the general result of all these 
actions being to increase the capacity of the 

Thus there are two ways in which the cavity 
of the thorax can be dilated; namely, by the 
action of the diaphragm, and by the action of 
the intercostal muscles. It is only in peculiar 
cogencies that the whole power of this appa- 
ratus is called into action ; for in ordinary res- 
piratioii the diaphragm is the chief agent em- 
ployed, and the principal effect of the action of 
the intercostal muscles is simply to fix the ribs, 


and thus giye greater purchajse to the diaphragm . 
The rauBcles of tlie ribs are employed chiefly to 
give active asslstaoce to the diaphragm, when, 
from any cause, a difficulty arises in dilating the 

In Birds Hie mechanism of respiration pro- 
ceeds upon a different plan, of which an idea 
may be derived from 'the view given of the lungs 

oi' the Ostt-ich, at L. L., Fig. 377. The construc- 
tion of the lungs of birds is such as not to admit 
of any change in their dimensions; for they are 
very compact in their texture, and are so closely 
braced to the ribs, and upper parts of the chest, 


by firm membranes, as to preclude all possi- 
l»l]ty of im)tion. They in part, indeed, project 
behind the intervals between the ribs, so that their 
whole mass is ncA altogether contained within 
the thoracic cavity. There is no large muscular 
diaphragm by which any change in the capacity 
of the chest could be effected, but merely a few 
narrow slips of muscles, arising from the inner 
sides of the ribs, and inserted into the thin trans- 
parent membrane which covers the lower surface 
of the lungs. They have the effect of lessening 
the concavity of the lungs towards the abdomen, 
at the time of inspiration, and thereby assist in 
dilating the air-cells*. The bronchia, or divisions 
of the trachea (t), after opening, as usual, into 
the pulmonary air-cells, do not terminate there, 
but pass on to the surface of the lungs, where 
they open by numerous apertures. The air is 
admitted, through these apertures, into several 
large air-cells (ccc), which occupy a consi- 
derable portion of the body, and which enclose 
most of the large viscera contained in the ab- 
domen, such as the liver, the stomach, and the 
intestinest ; and there are, besides, many lateral 
cells in immediate communication with the 

• Hunter in the Animal Economy, p. 78. 

t It was asserted by the Parisian Academicians, that the air 
got admission into the cavity of the pericardium, in which the 
heart is lodged. This error was Brst pointed out by Dr. Ma- 
cartney, in Ree8*8 Cyclop»dia.— Art. Bird, 


kmgs, and esttending all down ilie odes of tlie 
bodj. Numerous aif'-ceUs also exist lietweea 
the miiiscles, and ^todemeath the skin ; and the 
air penetraiei^ even into the interior of Ae bones 
themselves^ tilling' the i^aces nsimUy^ oeotrpied 
by the marrow, and thusfOontribiitinginatariaUy 
to the lightness of the fabric*. All these celfe 
are very large and numerous in birds whseh 
perform the high^ smd most rapid fligkkt, «uch 
as the eagle. The bill of the Tmican, whieh^is 
of a cellular dtructure, and sdso the cells betlr^^n 
the plates of the skull in the Owh are, in like 
manner, filled with' air ^derived from the lungs: 
the barrels of the large quills of the taUs and 
wings are dlso supplied with air from the same 

In birds, then, the air is not mer^y received 
into the lungs, but actually passes through them, 
b^irig drawn forwards by the Btiuscles of the ribs 
when they elevate the chesty and produce an 
expansion of the subjacent air^eeUs.' The chest 
is depressed, for the purpose of expiration, by 
another set of muscles^ and the air driven back ; 
this air, consequently, passes a ^aocmd time 
through the lungs, and acts twice on the blood 
which circulates in those organs. It is evident 
that if the lungs of birds had been constructed 

* In birds, not formed for exten&ive flight, as the gallinaceooB 
tribes, the humerus is the only bone into which air is introduced^ 
— Hunter on the Animal Economy, p. 81. 


on the pjan 'o£. those of quadrupeds, they m^ist 
ba^e been twice, as large to obtaia the same 
amoBiit ofaaratioii in tite blood; aild conse- 
<|aently must bave^^been twice as heavy, which 
wooldf^haTe b/Mn a serious, inconvenience in an 
animal &>m»d for flying"*. . The diffusion of so 
large a quantity of air throagfacwit tji^'bddy of 
animals of this class ; presents an analogy with 
a Bimite purpose apparent in the conformation 
of insects^ where the same ' object is effected by 

means of tracheae t« 

Thus has. the mechan^m of respiration been 
varied in the different classes of animals, and 
adapted to the particular dement, and mode of 
life designed for each. Combined with ' the 
peculiar mode of circidation, it affords a tole* 
rably accurate criterion of the energy of the 

* I must mention, however, that the correctness of this view 
of the subject is contested by Dr. Macartney, who thinks it 
probable that the air, on its return irom the large air-cells, passes 
dveotly by the large air-holes into the bronchia, and is riot 
brought a' second time into contact with the blood. 

t The peculiarities of structure in the respiratory system of 
birds have probably a relation to the capability we see them 
possess, of bearing with imptmity, very quiok and violent changes 
of atiBospheric pressure. Thus the Condor of the Andes i# 
often seen to descend rapidly from a height of above 20,000 
feet, to the edge of the sea, where the air is more than twice the 
density of &at which the bird had been breathing. We are as 
yet unable to trace the connexion which probably exists between 
the structure of the lungs, and this extraordinary power of accom- 
modation to such great and sudden variations of atmospheric 


vital pQwers. In birds, the muscular activity is 
raised to the highest degree, in consequence of 
the double effect of the air upon the whole cir- 
culating blood in the pulmonary organs. The 
Mammalia rank next below birds, in the scale 
of vital energy ; but they still possess a double 
circulation, and breathe atmospheric air. The 
torpid and cold-blooded reptiles are separated 
from mammalia by a very wide interval, because, 
although they respire air, that air only influences 
a part of the blood ; the pulmonary, being only 
a branch of the general circulation. In fishes, 
again, we have a similar result, because, al<- 
though the whole blood is brought by a double 
circulation to the respiratory organs, yet it is 
acted upon only by that portion of air which is 
contained in the water respired, and which is 
less powerful in its action than the same element 
in its gaseous state. We may, in like manner, 
continue to trace the connexion between the 
extent of these functions aiid the d^rees of 
vital energy throughout the successive classes of 
invertebrate animals. The vigour and activity 
of the Amotions of insects, in particular, have 
an evident relation to the effective manner in 
which the complete aeration of the blood is 
secured by the extensive distribution of tracheae 
through every part of their system. 


^ 4. Chemical Changes effected by Respiration 

We have next ta direct our attention to the che- 
mical offices which respiration performs in the 
ianimal economy. It is only of late years that 
we may be said to have obtained any accurate 
knowledge as to the real nature of this important 
function; and there is perhaps no branch of 
physiology which exhibits in its history a more 
humiliating picture of the wide sea of error in 
which the human intellect is prone to lose itself, 
when the path of philosophical induction is 
abandoned, than the multitude of wild and 
visionary hypotheses, devoid of all solid founda- 
tion, and perplexed by the most inconsistent rea- 
sonings, which formerly prevailed with regard to 
the objects and the processes of respiration. To 
give an account, or even a brief enumeration of 
these theories, now sufficiently exploded, would 
be incompatible with the purpose to which I 
must confine myself in this treatise.* I shall 

* For an account of the history of the various chemical 
theories which have prevailed on this interesting department of 
Physiology, I must refer to the '' Essay on Ilespiration,'* by Dr. 
Bostock, and also to the ^^ Elementary System of Physiology,*' 
by the same author, which latter work comprises the most com- 
jprehensive and accurate compendium of the science which has 
yet appeared. 


content myself, therefore, with a concise state- 
ment of such of the leading facts relating to this 
function, bs have now, by the labours of modern 
physiologists, been satisfactorily established, and 
which serve to elucidate the beneficent intentions 
of nature in the economy of the animal system. 
Atmospheric air acts without difficulty upon 
the blood while it is circulating through the 
vessels which are ramified over the membranes 
lining the air cells of the lungs ; for neither 
these membranes, nor the thin coats of the vessels 
themselves, present any obstacle to the trans- 
mission of chemical elements from the one to the 
other. The blood being a highly compound 
fluid, it is exceedingly difiicult to obtain an ac* 
curate analysis of it, and still more to ascertain 
with precision the difi*erent modifications which 
occur in its chemical condition at different times : 
on this account, it is scarcely possible to deter- 
mine, by direct observation, what are the exact 
chemical changes, which that fluid undergoes 
during its passage through the lungs ; and we 
have only collateral evidence to guide us in the 

* Some experiments very recently made by Messrs. Mcuatir^ 
and Marcet, on the ultimate analysis of arterial and venous 
blood, taken from a rabbit, and dried, have shown that the 
former contains a larger proportion of oxygen than the latter; 
and that the latter contains a larger proportion of carbon than 
the former : the proportions of nitrogen and hydrogen being the 


most obvious effect resulting from the ac- 
tion o€ the air is a change of colour from the dark 
purple hqe, which the blood has when it is brought 
to the hmgS) to the bright yacmillion colour^ 
which it is found to assume in those organs^ and 
which accompanies its restoration to the qualities 
of arterial blood. In what the chemical differ- 
ence between these two states ccmsists may, in 
some measure, be collected from the changes 
which the air itself, by producing them, has 

The air of the atmosphere, which is taken 
into the lungs, is known to comost of about 
tw^ity per cent, of oxygen gas, seventy-nine of 
nitrogen gas, and one of carbonic acid gasi 
When it has- acted upon the blood, and is re- 
turned from the lungs, it is found that a certain 
proportion of oxygen, which it had contained^ 
has disappeared, and that. the place of this 
oxygen is almost wholly supplied by an addition 
of carbonic acid gas, together with a quantity 
ctf watery vapour. It appears also probable that 
a small portion of the nitrogen gas is consumed 
during respiration. 

same in both. The following are the exact numbers expressive 
of these proportions: 

Carb&n, Oxxfgen, Nitrogen, Hydrogen, 

Acterial blood . . . 50.2 . . ..26.3. . . . 16.3 . . . 6j6 
Venous blood . . . 55.7 ... 21.7 .. . 16.2 ... 6.4 
Memoires de la Societe de Physique et d'Hist. Naturelle de 
Otnhe. T. v. p. 400. 


For our knowledge of the fact of tibe dis- 
appearance of oxygen we are indebted to the 
labours of Dr. Priestley. It had, indeed, been 
long before suspected by Mayow, that some 
portion of the air inspired is absorbed by the 
blood ; but the merit of the discovery that it is 
the oxygenous part of the air which is thus 
consumed is unquestionably due to Dr. Priestley. 
The exact quantity of oxygen, which is lost in 
natural respiration, varies indiiSerent animals, and 
even in different conditions of the same animal. 
Birds, for instance, consume larger quantities of 
oxygen by their respiration ; and hence require, 
for the maintenance of life, a purer air than 
other vertebrated animals. Vauquelin, however, 
found that many species of insects and wortos 
possess the power of abstracting oxygeii from 
the atmosphere in a much greater degree than 
the larger animals. Even some of the terres- 
trial moUusca, such as snails, are capable of 
living for a long time in the vitiated air in which 
a bird had perished. Some insects, which con- 
ceal themselves in holes, or burrow under ground, 
have been known to deprive the air of ev^jr 
appreciable portion of its oxygen. It is ob^ 
served by Spallanzani, that those animals, whoste 
modes of life oblige them to remain for a great 
length of time in these confined situa^<ms, 
possess this power in a greater degitee than 
others, which enjoy more liberty of moving in the 


open air: iKi admirably have the faculties of 
animals been, in every instance, accommodated 
to Jtheir respective wants* 

Since carbonic acid consists of oxygen and 
carbon, it is evident that the portion of that gas 
which is exhaled from the lungs is the result of 
the combioation of either the whole, or a part» 
of the oxygen gas, which has disappeared during 
the act of respiration, with the carbon contained 
HI the dark venous blood, which is brought to 
the lungs. The blood having thus parted with 
k» superabundant carbon, which escapes in the 
Ibrm of carbonic acid gas, regains its natural ver- 
million colour, and is now qualified to be again 
transmitted to the diflferent parts of the body for 
their nourishment and growth. As the blood 
ccbtains a greater proportion of carbon than the 
animal solids and fluids which are formed from 
it, this supembundant carbon gradually accu- 
mulates in proportion as its other principles, 
(namdy, oxygen, hydrogen, and nitrogen) are 
abstracted from it by the processes of secretion 
and nutrition.. By the time it has returned to 
the heart, therefore, it is loaded with carbon, 
a princ^le, which, when in excess, becomes 
noxious, and requires to be removed from the 
blood, by combining it with a fresh quantity of 
oxygen obtained^from the atmosphere. It is not 
yet satisfiEu^torily determined whether the whole 

VOL, u. z 



of the oxygen, which disappean during respi- 
ration, is employed in the formation of carbonic 
acid gas: it appears probable, howeyer, fironK 
the concurring testimony of many experiHien- 
talists, that a small quantity is permanently 
absorbed by the blood, and enters into it as one 
of its constituents. 

A wnilar question arises with respect to 
nitrogen, of which, as I have already mentioned^ 
it is probable that a small quantity disaf^ieam 
from the air when it is respited ; althoqgh the 
accounts of experimentalists are not uniform on 
this point. The absorptimi of nitrogen during 
respiration was one of the results which Dr. 
Priestley had deduced from his exp^ments: and 
this fact, though often doubted, appears, on the 
whole, to be tolerably well ascertained by the 
inquiries of Davy, Pfaff, and Henderson. With 
regard to the respiration of cold-blooded animals^ 
it has been satisfieu^torily established by the 
researches of Spallanzani, and more especially 
by those of Humboldt and Provencal, on fishes, 
that nitrogen is actually abswbed.. A confinna- 
tion of this result has recently been obtained by 
Messrs. Macaire and Marc^ who have found 
that the blood contains a larger pr<^rtioii of 
nitrogen than the chyle, from which it is formed. 
We can discover no other source from which 
chyle could acquire this additional quantity of 
nitrogen, during its conversion into blood, thaii 


the air of the atmosphere, to which it id exposed 
in its passage through the pulmonary vessels.^ 

According to these views of the chemical 
objects of respiration, the process itself is ana* 
logons to those artificial operations which effeict 
the combustion of charcoal. The food supplies 
the fuel, which is prepared for use by the di- 
gestive oiigans, and' conveyed by the pulmonary 
aMenes to the place where it is to undergo com- 
bustion : the diaphra^ is the bellows, which 
feeds the furnace with air; and the trachea is 
the chimney, through which the carbonic acid, 
wMch is the product of the combustion, escapes. 

It becomes an interesting problem to deter:, 
mine whether this anal<^ may not be further 
extended ; and whether the combustion of car- 
bon, which takes place in respiration, be not the 
exclusive source of the increased temperature^ 
which all animals, but more especially those 
designated as warm-blooded, usually maintain 
above the surrounding medium. The uniform 
and exact relation which may be observed to 
take- place between the temperature of animals 
and the energy of the respiratory function, or 
rather the amount of the chemical changes 
induced by that function^ affords very strong 
eyidence in fiivour of this hypothesis. The 
cmncidence, indeed, is so strong, that notwitb- 
Btanding the objections that have been raised 

* See tb^ note at page 334. 


against the theory founded upon this hypo* 
thesis, from some appar^it anomalieB which 
occasionally present themselves, we must, I 
think, admit that it affords the best expluiation 
of the phen(Mnena of any theory yet proposed^ 
and that, therefore, it is probably the true one. i 
The. maintenance of a very elevated temper 
rature appears to require the coDcurrence of twp 
conditions ; namely, first, that the whole of the 
blood should be subjected to tl)e influence of the 
air, and, secondly, that that air should be pref 
sented to it in a gaseous state. These, then, are 
the circumstances which estaUish the great dis>>^ 
tinction between warm and cold-blooded anknals ; 
a distinction which at once stamps the diaracter 
of their whc^e constitution. It is the conditieii 
of a high temperature in the blood which raiseift 
the quadruped and the bird to a rank, in the 
scale of vitality, so far above that of the reptiles 
it is this which places an insiqierable boundary 
between mammalia and fishes. However the 
warm-blooded Cetacea, who spend their lives 
in the ocean, may be found to approximate 
in their outward form, and in their external 
instruments of motion, to the other mhabitaBto 
of the deep, they are still, from the conformatioii 
of their respiratory organs, dependent on another 
element. If a seal, a porpoise, or a dolphin 
were confined, but for a short time, under the 
surface of the water^ it would perish with the 


same certainty as any other of the mammalia, 
placed in the same situation. We observe them 
continually rising to the surface in order to 
breathe, under erery circumstance of privation 
or of danger ; and however eagerly they may 
pursue their prey, however closely they may be 
pressed by their enemies, a more urgent want 
compels them, from time to time, to respire air 
at the surface of the sea. Were it not for this 
snpeiious necessity, the Whale, whose enormous 
bulk is united with corresponding strength and 
Bwiftness, would live in undisturbed possession 
4^ the widely extended domains of the ocean^ 
mj^t view without dismay whole fleets sent out 
against him, and might defy all the efforts that 
man could practise for his capture or destruction. 
But the constitution of bis blood, obliging him 
to breathe at the surface of the water, brings 
him within the reach of the fatal harpoon. In 
rain, on feeling himself wounded^ does he plunge 
lor reluge into the recesses of the deep ; the sam^ 
necessity recurs, and compelling him again to 
present himself to his foes, exposes him to their 
renewed attacks, till he falls in the unequal 
fltro^^le. His colossal form and gigantic strength 
are of little avail against the power of man, feeble 
Uiough that power may seem, when physically 
considered, but which derives resistless might 
from its association with an immeasurably su- 
perior intellect. 


Chapter XII, 


The capability of effecting certain chemical 
changes in the crude materials introduced into 
the body, is one of the powers which more espe^ 
ciiedly characterize life ; but although this power 
is exercised both by vegetable and by animal 
organizations, we perceive a marked difference 
in the results of its operation in these two orders 
of beings. The food of plants consists, f<Mr Afi 
most part, of the simpler combinations of ele-^ 
mentary bodies, which are elaborated in cellular 
or vascular textures, and converted into various 
products. The oak, for example, forms, Vy the 
powers of vegetation, out of these elements, not 
only the green pulpy matter of its leaves, and 
the light tissue of its pith, but also the densest of 
its woody fibres. It is from similar materials, 
agairi, that the olive prepares its oil, and tbe 
cocoa-nut its milk ; and the very same elements, 
in different states of combination, compose, in 
other instances, at one time the luscious sugar 
of the cane, at another the narcotic juice of the 
poppy, or the acrid principle of the euphorbium ; 
and the same plant which furnishes in one part 


the bland farina of the potatoe, will produce in 
another the poisonous extract of the nightshade.* 
Yet all these, and thousands of other vegetable 
products, differing widely in their seni^ble quaU- 
ties, agree very nearly in their ultimate chemical 
analysis, and owe their peculiar properties chiefly 
to the order in which their :elements are arranged ; 
an order dependent on the processes to which 
they have been subjected in the system of each 
partipttlar vegetable. 

In the animal kingdom we observe these pro* 
cesses multiplied to a still greater extent ; and 
the resulting substances are even farther removed 
fr<»n the original condition of unorganized matter. 
In the first place, the food of animals, instead 
of being simple, like that of plants, has always 
undergone previous preparation ; for i^ has 
either, constituted a portion of some other organ- 
ised being, or it has been a product of organiza- 
tion ; in each case, therefore, partaking of the 
cmnplexity of composition which characterises 
dganized bodies. StUl, whatever iQay be its 
qualities when received into the stomach, it is 
soon converted by the powers of digestion into 
a milky, or transpar^t fluid, having nearly the 
same uniform properties. We have seen that 
there is scarcely any animal or vegetable sub- 
Blance, however dense its texture, or virulent its 
qualities, but is capable of affording nowish- 
ment to various species of animals. Let us take 


BB an example the elytra of cantharides, which 
are such active stimnlants when applied in 
powder to the skin in the ordinary mode of 
Uistering; we find that, notwithstanding their 
highly acrid qualities, they ciHistitute the natural 
food of several species of insects, which devour 
them withgreat avidity; and yetthefluids of these 
insects^ though derived from this pungent food, 
are perfectly bland, and devoid of all acrimony*^ 
Cantharides are also, according to Pallas, die 
fovourite food of the hedge-hog; although to 
other mammalia they are highly poisonous. It 
has also been found that even those animal 
secretions, (such as the venom of the rattle* 
snake,) which, when infused, evenin the minutest 
quantity, into a wound, prove instantly fatal, 
may be taken into the stomach without produce 
ing any deleterious effects. These, and a mul-^ 
titude of other well-known facts, fully prove 
how completely substances received as aliment 
may be modified, and their properties changed^ 
or even reversed, by the pow'ers of animal 

No less remarkable are the transmutatkna^ 
which the blood itself, the result of these pre- 
vious processes, is subsequently made to undei^o 
in the course of circulation, and whesi subjected 
to the action of the nutrient vessels and secrefef 
ing organs ; being ultimately converted into the 


varuns textures and substances which codiposQ 
all the parts of the frame. All the modifications 
of cdkdar sabstance, in ifes various statics of con- 
densation; the membranes, the ligaments, th6 
cartilages, the bones, the marrow ; the inuacled, 
with their tendons ; the lubricating fluid of the 
jmnts; the medullary pulp of the brain; the 
transparent jdly of the eye ; in a word, all the 
fiveraified textures of the various organs, which 
are calculated for such different offices, are 
derived from the same nutrient fluid, and may 
be considered as being merely modified arrange- 
ments of ttie same ultimate chemical elements. 

In what, then, we naturally ask, consists 
this subtle diemistry of life, by which nature 
eSSectB these multifarious changes ; and in what 
secret recesses of the living frame has she con- 
structed the refined laboratory in which she 
operates her marvellous transformations, far sur- 
passing even those which the most visionary 
alchemist of former times had ever dreamed of 
achieving? Questions like these can only be 
fairly met by the confession of profound igno- 
rance ; for, although the subject of secretion has 
long excited the most ardent curiosity of phy(»- 
ologists, and has been prosecuted with extraor- 
dinary zeal and perseverance, scarcely any 
positive information has resulted from their 
labours, and the real nature of the process 


remains involved nearly in the same degree of 
obscurity as at first.* It was natural to expect 
that in this inquiry material assistance would be 
derived from an accurate anatomical examina- 
tion of the organs by which the more remarkable 
secretions are formed ; yet, notwithstanding tii» 
most minute and careful scrutiny of these organs^ 
our knowledge of the mode in which they are 
instrumental in effecting the operations whtdi 
are there conducted, has not in reality advanced a 
single step. To add to our perplexitjr we often 
see, on the one hand, parts, to all appearance 
very differently organized, giving rise to secre* 
tions of a similar nature ; and on the other hand, 
substances of very different properties produced 
by organs, which, even in their minutest details, 
appear identical in their structure. Secre* 
tions are often found to be poured out from 
smooth and membranous surfaces, such as those 

* It is not yet precisely determined to what extent the organs 
of secretion are immediately instrumental in producing the sub* 
stance which is secreted ; and it has been even suggested that 
possibly their office is confined to the mere separation, or fittra* 
tion lirom the blood, of certain animal products, whieh art 
always spontaneously forming in that fluid in the course of its 
circulation. This hypothesis, in which the glands, and other 
secreting apparatus are regarded as only very fine strainers, is 
supported by a few facts, which seem to indicate the presence of 
these products in the blood, iqdependently of the secMiag 
processes by which they are usually supposed to be formed; but 
the evidence is as yet too scanty and equivocal to warrant the 
deduction of any general theory on the subject. 

fiECRSTlOK. 347 

which line the cavities of the abdomen, the chest, 
and the head, and which are also reflected in- 
wards so as to invest the organs therein contained, 
tm the hearty the lungs, the stomach, the intestines, 
the liver, and the brain*. In other instances, 
the secreting membrane is thickly set with 
minute processes, like the pile of velvet : these 
processes are called villi^ and their more obvious 
Use, as far as we. can perceive, is to increase the 
surface from i^rhich the secretion is prepared. 
At other times we see an opposite kind of struc- 
ture employed ; the secreting surface being the 
int^ual lining of sacs or cells, either opening at 
once into some larger cavity, or prolonged into 
a tube, or duct, for conveying the secreted fluid 
to a more distant point. These cells, ox follicles^ 
as they are termed, are generally employed for 
the mucous secretions, and are often scatteied 

* Sometimes the secreting organ appears to be entirely com- 
poied of a mass of vessels covered with a smooth membrane ; 
in other cases, it appears to contain vome additional material, or 
parenchyma, as it is termed. Yertebrated animals present m 
with numerous mstances of glandular organs em]^oyed for special 
{Nirposes of secretion : thus, in the eyes of fishes there exists a 
large vascular maiss, which has bee^ called the choroid glands 
and which is supposed to be placed there for the purpose of 
replenishing some of the humours of the eye, in proportion as 
they are wasted. Within the air-bladder of several species of 
fishes there is found a vascular o^an, apparently serving to secrete 
the air with which the bladder is filled ; numerous ducts, filled 
with air, having been observed proceeding from the organ, and 
opening on the inner surface of the air-bladder. • 


throughout the surfaccis of membraneB* : at oAfer 
times the secretiug cayiti^s are collected in gteat 
numbers into groups ; and they then frequently 
consist of a series of length^ed tubes, likecsocf^ 
examples of which we have already seen in tile 
hepatic and salivary glands of insectd. 

A secretory organ, in its simplest form, cdnr 
sists of short, harrow smd undivided tubes ; we 
next find tubes which are elongated, tortuous or 
convoluted, occasionally presenting dilated p(Mv 
tions, or eveii having altogether the appearance 
of a collection of pouches, or sacs ; while in other 
cases, they are branched, and extend into muimt^ 
ramifications. Sometimes they are detached^ or 
isolated ; at other times they are collected into 
tufts, or variously grouped into masses, where stall 
the separate tubes admit of being unravelled* The 
secreting filaments of insects float in the general 
cavity, containing the mass of nutrient fluid, and 
thence imbibe the materials they require for the 
performance of their functions* It is only when 
they receive a firm investment of cellular mem* 
brane, forming what is termed a capsule^ and 
assuming the appearance of a compact body» 
that they properly constitute a gland; and this 
form of a secreting organ is met with only among, 
the higher animalsf* 

* See p. 185 of this volume; and in particular Fig. 305. 
Sebaceous follicles are also noticed in Vol. i. p. 114. 

t Dr. Kidd, however, describes bodies apparently of a giaii* 
dular character, disposed in rows on the inner surface of the 


Great variety is observable both in the form and 
structnre of different glands, and in the mode in 
which their blood-vessels are distributed. In 
anjmala which are furnished with an extensive 
birculatidn, the vessels' supplying the glands with 
blood aie distributed in various modes ; and it 
\A evident thai each plan has been designedly 
selected with reference to the nature of the par^ 
ticular secreticm to be performed, although we 
are here unable to follow the connexion between 
the pieans and the ^d. In some glands, ioit 
duf aniple, the minute arteries, on their arrival at 
the QVgan, suddtHily divide into a great number 
of smaller branches, like the fibres of a camel- 
hair pencil : this is called the penicillated struc- 
ture. Sometimes the minute branches, instead 
of proceeding parallel to each other after their 
division, separate like rays from a centre, pre- 
senting a stellated^ or star-like arrangement. In 
the greater number of instances, the smaller 
arteries take a tortuous course, and are some- 
times coiled into spirals, but generally the con- 
volutions are too intricate to admit of being 
imravelled. It is only by the aid of the micros- 
cope that these minute and delicate structures 
can be rendered visible; but the fallacy, to 
which all observations requiring the application 
of high mi^nifying powers are liable, is a serious 

intestinal canal of the Oryllotalpot or mole«>cricket. Phil. Tran. 
for 1825, p. 227. 


obstacle to the adyancement of our knowledge 
in this department of physiology. Almost the 
only result, therefore, which can be collected 
from these laborious researches in microscc^ic 
anatomy, is that nature has employed a great 
diversity of means for the accomplishment of 
secretion ; but we still remain in ignorance as to 
the kind of adaptation, which must assuredly 
exist, of each structure to its respective object, 
and as to the nice adjustment of chemical affinities 
which has been provided in order to accomplish^ 
the intended effects*. Electricity is, no. doubt, 
an important agent in all these processes ; but 

* The only instance in which we can perceive a correspondence . 
between the chemical properties of the secretion, and the kind 
of blood from which it is prepared, is in the liver, which, unlike 
all the other glands, has venous, instead of arterial blood, sent 
to it for that purpose. The veins, which return the blood that 
has circulated through the stomach, and other abdominal viscera, 
are collected into a large trunk, called Che venaportm<, which 
enters the liver, and is there again subdivided and ramified, as if 
it were an artery : its minuter branches here unite with those 
of the hepatic artery, and ramify through the minute lobules 
which compose the substance of the liver. After the bile is 
secreted, and carried off by hepatic ducts, the remaining Mood 
is conducted, by means of minute hepatic veins, which occofij 
the centres of each lobule, into larger and larger trunks, till they 
all unite in the vena cava, going directly to the heart. (See 
Kieman's Paper on the Anatomy and Physiology of the livo*, 
Phil. Trans, for 1833, p. 711.) A similar system of venouf 
ramifications, though on a much smaller scale, has been dis- 
covered by Jacobson, in the kidneys of most fishes and iqptiles, 
and even in some birds. 


in, the absence of all certain knowledge as to 
the mode in which it is excited and broaght into 
play in the living body, the chasm can for the 
present be supplied only by remote conjecture. 

The process which c<mstitutes the ultimate 
stage of nutrition, or the actual incorporation of 
the new material with the solid substance of the 
body, of which it is to form a part, is involved 
in equal obscurity with that of secretion. 



Absorption is another function, related to nutri- 
tion, which deserves special notice. The prin* 
cipal object of this function is the removal of 
such materials as have been already deposited, 
and have become either useless or injurious, and 
th^r conveyance into the general mass of circu- 
lating fluids ; purposes which are accomplished 
by a peculiar set of vessels, called the Lym- 
phatics. These vessels contain a fluid, which 
being transparent and colourless like water, 
bas been denominated the lymph. The lym- 
phatics are perfectly similar in their structure, 
and probably also in their mode of action, to 
the lacteals, which absorb the chyle from the 


intestinal cavity: they aie found in all the. 
dasses of vertebrated animals, and pervade 
extensively every part of the body. Exceed-^ 
ingly minute at' their origin, they unite toge- 
ther as they ptoceeA, forming kocg^r and 
larger trui^ks, generally following the course of 
the veins, till they finally discharge their con-, 
tents either into the thoracic duct, or into some 
of the large veins in the vicinity of the hearts 
Throughout their whole course they are, like the 

lacteals, provided with numerous 
valves, which, when the vessel is dis- 
tended with lymph, give it a resem- 
blance to a string of beads, Fig. 378.* 
In the lower animals it appears that 
the veins are occasionally endowed 
with a power of absorption, similar 
to that possessed by the lymphatics. None of 
the invertebrata, indeed, possess lymphatics, and 
absorption must consequently be performed by 
the veins, when these latter vessels exist. The 
addition of the system of lymphatic vessels, ba 

* In warm-blooded animals, the lymphatics are made to 
traverse, in some part of their course, oen^ain bodies of a 
compact structure, resembling glands, and termed accordingly, 
the lymphatic glands. One of these is represented in Fig. 378. 
They correspond in structure, and probably also in their functions, 
to the mesenteric glands, through wjiich, in the mammalia, the 
lacteals pass, before readiing the thoracic duct. It is chiefly in 
the mammalia, indeed, &at these glands are met with, for they 
are rare among' birds, and still more so among fislies and 

ABSOft^ION. 353 

auxiliaries to the yeins, may therefore be re- 
garded as a refinement in organization, pecnliar 
to the higher classes of animals^. 

Professor MuUer, of Bonn, has lately disoo- 
Tered that the firog, and sereral ether amphibious 
animals, are provided with large receptacles for 
die lymph, situated immediatdiy under the skin, 
and exhibiting distinct and regular pulsations, 
Ifte the heart. The use of these lymphatic 
hearts^ as they may be called, is evidently to 
propel the lymph in its proper course along the 
lymphatic vessels. In the frog four of these 
Mgans have been found ; the two posterior hearts 
being situated behind the joint of the hip, and the 
fwo anterior oiies on isach side of the transverse 
process of the third vertebra, and under the 
posterior extremity of the scapula. The pulsa** 
tions of these lymphatic hearts do not correspond 
with those of the sanguiferous heart; nor do 
those of the right and left sides take place at 
the same time^, but they o£fcen alt«nate in am 
irr^ukur manner. ProfesBor MuUer has disco* 
vered sioftflar oigans in the toad, the salamander, 
and the green lizard, and tlnnki it probable that 
they exist in all the ampUbiaf. 

• Fohmann, who has made extensive researches on the ab- 
sortieht vessels throughout all the classes of vertebrated ariitnals', 
kas feund that th^ termmate extensively in the veins. See his 
work, entitled *' Anatomiiche Untersuehungen tber die Ver- 
Unduttg der Saugadem mit den Venen." 

t Phil. Trans, for 1833, p. 89. 



Chapter XIV. 


The organs which are appropriated to the per- 
formance of the yariouB functions conducive to 
nutrition, are generally designated the vital 
argansy in order to distinguish them from those 
which are subservient to sensation, volun- 
tary motion, and the other iunctions of animal 
life. The slightest reflecticm on the variety and 
complication of actions comprised under the 
former class of functions in the higher animab, 
will convince us that they must be* the result 
of the combined operation of several different 
agents ; but the principal source of mechanical 
force required by the vital organs, is still, as in 
all other cases, the muscular power. The coats 
of the stomach and of the intestinal tube coixtain 
a large proportion of muscular £bres, the con- 
tractions of which effect the intermixture and 
propulsion of the contents of these cavities, in 
the manner best calculated to favour the che- 
mical operations to which they are to be sub- 
jected, and to extract from them all the nourish- 
ment they may contain. In like manner, all 


the tubular vessels, which transmit fluids, are 
endowed with muscular powers adapted to the 
performance of that office. The heart is a strong 
hollow muscle, with power adequate to propel 
the blood, with immense force, through the 
arterial and venous systems. The blood-vessels, 
also, especially the minute, or capillary arteries, 
besides being elastic, are likewise endowed with 
muscular power, which contributes its share in 
forwarding the motion of the blood, and com- 
pleting its circulation. The quantity of blood 
circulating in each part, the velocity of its 
motion, and the heat which it evolves, are 
regulated in a great measure by the particular 
mode of action of the blood-vessels of that part. 
The quantity, and sometimes even the qua- 
lity of the tecretions, are dependent, in like 
manner, on the conditions of the circulation; 
and the action of the ducts, which convey the 
secreted fluids to their respective destinations, is 
also resolvable into the eflects of a muscular 

The immediate cause which, in th^se organs, 
excites the muscular fibre to contraction, may 
i^requently be traced to the forcible stretching of 
its parts. This is the case in all hollow and 
tubular muscles, such as the stomach, the heait, 
and the blood-vessels, when they are mechani- 
cally distended, beyond a certain degree, by the 
presence of contained fluids, or other substances. 


At other times, the chemical quality of their 
contents appean to be * the immediale sfcimiilw 
inciting them to oontraetion. But, msmamiA in*^ 
stances occur, in die higher orders oi animals^ 
in which these causes alone are inadequate to 
explain the phenomena of the vital fundionsi 
No mechanical hypothesis will suffice to account 
for the infinite diversity in. the modes of actioo 
<^ the organs which perform: the» functions^ . or 
afford any clue ta the. means by whi^h they aoe 
made to co-operatev with. such nicety of adjusts 
ment, in the ^production of the ultimate effect 
Still less will anjr [theory^ comprising only the 
agency, of ^he muBcular power, and the wdinaiy 
chemical affinities, enable us to ea^lain how w 
irritating cause, applied at one part^ diall pro- 
duce. £ts visible effibcta oi> a distant cHrgan ; or im 
what way remote and apparently unconneeted 
parts shall, as if by an invisHble sympatby^ be 
brought at the same monlent to act in concert, 
in the production of a common effect. Yet fmtk 
co-operation must, in innumerable cases, be 
absolutely indispensable to the perfect accom- 
pUshment of the vital functions of animals. 

Nature has not ne^ected objects 6o important 
to the success of her measures, but . has pro- 
vided, for the accomplishmi^it of these purposes, 
a controlling faculty, residing in the nervous 
system, and denominated the . nervous pawm. 

n£«vjous power. 357 

Ei!f)erii&ents have diown that the due perform- 
ance of the vital limctioba of digeistion, of circa- 
latioKi, aad of seer^ion, requires the presence of 
an agency, derived from different parts of the 
brain and spinal marrow, and regulating the 
order and combinations of the actions of the 
organs which are to perform those functions. 
The same influence, for example, which in- 
creases* the ptswor of secsetion in any particular 
^an4i is found .to increase, at the same time, 
the action of those blood- vessds which supply 
that gland with the materials for secretion ; and 
conversely^ the increased action of the blood- 
vessels is Accompanied by an increased activity 
ei the- 9eorating <»fgan. Experience also shows 
that when .the influenoe of the brain and spinal 
marrow is intercepted^ although the afflux of 
blood may ^ for a time, continue,' yet the secretion 
•cetseSf: vid 9II the functions dependent upon 
secretipn, such a» digestion, cease likewise. 
'Th«s^4he QOrvousr -power combines together dif- 
ferent operations, Adjusts their reispective de- 
^grees,. and -regulates their succession, so as to 
ensure that perfect harmotity which is essential 
to the attainment of the objects of the vital func- 
tions; ajid thus, not only the muscular power 
wldeh . resides in the vital organs, but also the 
organic affinities which produce secretion, and 
all those unknown causes which effect the nutri- 


tioD, developement, and growth of each part, are 
placed tmder the control of the nerrous power.* 
Although we are entirely ignorant of the na- 
ture of the nervous power, we know that, when 
employed in the vital functions, it acts through 
the medium of a particular set of fibres, which 
form part of the nervous system, and are classed, 
therefore, among the nerves. The principal 
filaments of this class of nerves compose what 
is called the sympathetic nerve^ from its being 
regarded as the medium of extensive sympathies 
among the organs ; but the whole assemblage of 
these nerves is more commonly known by the 
name of the ganglionic system^ fit>m the circum- 
stance of their bdng connected with small masses 
of nervous substance, termed ganglia^ which are 
placed in difierent parts of their course. Fig. 
379, represents a ganglion (g), through which 
the nerve (n), consisting at its origin of a number 
of separate filaments (f), is seen to pass, before 
it subdivides into branches (b). The numerous 
communications and interchanges of filaments, 
which subsequently take place at various parts, 
forming what is called a plexus^ are shown in 

* As the Amotions of plants are sufficiently simple to admit 
of being conducted without the aid of muscular power, still less 
do they require the assistance of the nervous energy : both of 
which properties are the peculiar attributes of animal vitality. 
We accordingly find no traces either of nervous or of muscular 
fibres in any of the vegetable structures. 



'ig. 380 : where four trunks (t, t) divide into 
branches, . which are again separated, and va- 

riously reunited in their course, like a ravelled 
skein of thread, before they proceed to their 
respective destinations. 

The ganglia are connected by nervous fila* 
ments with every part of the brain and spinal 
marrow, the great central organs of the nervous 
system; and they also send out innumerable 
branches, to be distributed all over the body. 
All the parts receiving blood-vessels, and more 
especially the organs of digestion, are abun- 
dantly supplied with ganglionic nerves ; so that, 
by their intervention, all these parts have ex- 
tensive connexions with the brain and. spinal 
marrow, and also with one another. The ganglia 
are more particularly the points of union between 
nervous fibres coming from many difierent parts : 
they may be considered, therefore, as performing, 
with r^ard to the vital functions, an office anar 


logons to that which die brain and spinal mtarrmr 
perform with rc^^aid to the other nerves, or as 
being secondary centres of nervous power. Thns 
there are two important objects for which the 
nerves belonging to the ganglionic system have 
been provided; first, to serve as the channels 
through which the affectiouB of one organ might 
be enabled to influence a distant organ; and 
secondly, to be the medium through which the 
powers of several parts might be combined and 
concentrated for effecting particular purposes, 
requiring such co-operation. Hence it is by 
means of the ganglionic nerves that all the 
organs and all the functions are rendered effi- 
cient in the production of .a common object, and 
are brought into one ccnnprehensive and har- 
monious system of operation. 

The nervous power, the effects of which we 
are here considering, should be carefully dis- 
tinguished from that power which is an attribute 
of another , portion of the nervous system, and 
idiich, being connected with sensation, volition, 
qnd other intdlectiial operations, has been deno- 
minated sensorial power J'^ The functions of di« 
gestion, circulation, ' absorption, secretion, and 
all those included under thb class of nutrient or 
vital fiinctions, are carried on in secret, are not 

* This distinction has been most clearly, pointed out, and illus- 
trated by Dr. A. P. W. Philip. See his ** Experimental Inquiry 
into the Laws of the Vital Functions/* 



necessarily, or eyen usually attended with sen- 
sation, and are wholly removed from the control 
of volition. Nature has not permitted processes, 
which are so important to the preservation of 
life, to be in any way interfered with by the will 
of the animal. We know that in ourselves they 
go on as well during sleep as when we are 
awake, and whether our attention be directed 
to them or not ; and though occasionally in- 
fluenced by strong Amotions, and other affections 
of mind, they are in general quite independent 
of every intellectual procesii* In the natural and 
healthy condition of the system all its internal 
o{>erations proceed quietly, steadily, and con- 
fittuitly, whether the mind be absorbed in thought 
or whc^y vacant. The kind of existence result- 
ii% firom these functions alone, and to which our 
itttetftfon hte hitherto been confined, must be 
iegard€id as the result of mere .vegetative, railier 
ttan' of animal life. It is time that we turn our 
views 16 the higher objects, and more curious 
field ^ isfquiry, belongin'g to the latter< 

t ; 





The system of mechanical and chemical func-* 
tions which we have been occupied in reviewing^ 
has been established only as a foundation for. 
the endowment of those higher faculties whicbi 
constitute the great objects of animal existence^ 
It is in the study of these final purposes that 
the scheme of nature, in the formation of the 
animal world, opens and displays itself in all its 
grandeur. The whole of the phepomena we 
have hitherto considered concur in one esaaitial 
object, the maintenance of a simply vital exists 
etice. Endowed with these properties alone, the 
organized system would possess all that is abso* 
lutely necessary for the continuance and support 
of mere regetatiye life. The machinery pro* 
vided for this purpose is perfect and complete in 
all its parts. To raise it to this perfection, not 


only has the Divine Architect emjdoyed all the 
properties and pow^s of matter, which science 
has yet lefealed to man, but has also brought 
into play the higher and more mysterious ener- 
gies of nature, and has made them to concur in 
the great work that was to be performed. On 
the organized fabric there has been conferred a 
vital force ; with the powers of mechanism have 
been conjoined those- of chemistry ; and to these 
have been superadded the still more subtle and 
potent agencies of caloric and of electricity : every 
resource has been employed, every refinement 
studied, every combination exhausted that could 
ensure the stability, and prolong the duration of 
the system, amidst the multifarious causes which 
continually menace it with destruction. It has 
been supplied with ample means of repairing the 
acddehts to which it Is ordinarily exposed ; it 
has been protected from the injurious influence 
of the surrounding elements, and fitted to resist 
for a lengthened period the inroads of disease, 
and the progre&s of decay. 

But can this, which is mere physical exist- 
aice, be the sole end of life ? Is there no far- 
ther purpose to be answered by structures so 
exquisitely contrived, and so bountifully pro- 
vided with the means of maintaining an active 
existence, than the mere accumulation and co- 
hesion of inert materials, differing from the 
stones of the earth only in the more artificial 


arrangement of their particles, and the more 
varied configaration 4>f their texture? Is the 
grorwth of an animal to be ranked in the same 
class of pfaeaiomena as the ooncretion of a pebble^ 
or the crystallization of a salt ? Must we not 
ever associate the power of feeling with the idea 
of animal life ? Can we direst ourselVes of the 
persua»on that the movements of animak, di* 
rected, like our own^ obvioiis.eiidsj proceed 
from voluntary acts, and imply the opeiation of 
an intellectv not wholly dissimilar in its spiiittial 
essa^ce from our own ? In vais may Descartes 
and his followers labour to siistain their paradox^ 
that brutes are only automata, — mere pieces of 
artificial mechanism, insensible either to plea- 
sure or to. pain, and incapable of internal .aifec' 
tiona, aiialogous to those of which, we are con* 
scious in ourselves. Their sophistry will avaU 
but little against the plain dictates of the umdert 
standing. To those who refuse to admit that 
enjoyment, which implies the powers of sensa- 
tion, and of voluntary jnotton, is the great end of 
animal existence,; the object of its creation must 
for ever ^remain a dark and impenetrable mys«> 
<tery; by such minds most all iurther inquiry 
into final causes be at once afoandcned as utterly 
vain and hopdess* But it surely requires no 
4aboured refutation to overbuti a system that 
violates every analogy by which our reasontngB 
on these subjects must necessarily be guided; 


and no artificial logic or scholastic syllogisms 
will long preyail bver the natural sentiment^ 
which must ever guide our conduct, that animals 
possess powers of feeling, and of spontaneous 
action, and faculties appertaining to those of 

The functions of sensation, perception, and 
voluntary motion require the presence of an 
animal substance, which we find to be organized 
in a peculiar manner, and endowed with very - 
remarkable properties* It is' called the meditl- 
iary substance ; and it composes the greater part 
of the texture of the brain, spinal marrow, and 
nerves; organs, of which the assemblage is 
known by the general name of the nervous system*, 
Certain afiections of particular portions of this 
medullary substtfnce, generally occupying som^ 
central situation, are, in a way that is totally 
inexplicable, connected with affections of the 
sradient and intelligent principle ; a jHinciple 
which we cannot any otherwise conceive than as 
being distinct from matter; sdthough we know 
that it is capable of being affected by matter 
operating through the medium of this nervous 
substance, and that it ts capable of reacting 
upon matter through the same medium. Of the 
truth of these propositions there exist abundant 
proofii; but as the evidence which establisAies 
them will mare conveniently come under our 
notice at a 9ubsequent period of our inquiry, I 


shall postpone their consideration ; and, proceed- 
ing upon the assumption that this connexion 
exists, shall next inquire into the nature of the 
intervening steps in the process, of which sen^- 
sation and perception are the results. 

Designating, then, by the name of brain this 
primary and essential organ of sensation, or the 
organ whose physical affections are immediately 
attended by that change in the percipient being 
which we term sensation; let us first inquire 
what scheme has been devised for enabling the 
brain to receive impressions from such external 
objects, as it is intended that this sentient being 
shall be capable of perceiving. As these objects 
can, in the first instance, make impressions only 
on the organs situated at the surface of the 
body, it is evidently necessary that some medium 
of communication should be provided between 
the external organ and the brain. Such a me- 
dium is found in the nei*ves^ which are white 
cords, consisting of bundles of threads or fila- 
ments of medullary matter, enveloped in sheaths 
of membrane, and extending continuously ftom 
the external organ to the brain, where they aH 
terminate. It is also indispensably requisite 
that these notices of the presence of objects 
should be transmitted instantly to the brain ; fot* 
the slightest delay would be attended with se<* 
nous evil, and might even lead to fatal omse- 
quences. The nervous power, of which, in our 


leyiew of the vital functions, we noticed some of 
the operations, is the agent employed by nature 
for this important office of a rapid communica- 
tion of impressions. The velocity with which 
the nerves subservient to sensation trsmsmit the 
impressions they receive at one extremity, along 
their whole course, to their termination in the 
brain, exceeds all measurement, and can be 
compared only to that of electricity passing 
along a conducting wire. 

It is evident, therefore, that the brain requires 
to be furnished with a great number of these 
nerves, which perform the office of conductors of 
th^ subtle influence in question ; and that these 
nerves must extend from all those parts of the 
body which are to be rendered sensible, and 
must unite at their other extremities in that 
central organ. It is. of especial importance that 
the surface of the body, in particular^ should 
communicate all the impressions received from 
the contact of external bodies, and that these 
impressions should prqduce the most distinct 
perceptions of touch. Hence we find that the 
skin, and all those parts of it more particularly 
intended to be the oigans of a delicate touch, 
are most abundantly supplied with nerves ; each 
nerve, however, communicating a sensation dis* 
tinguishable from that of every other, so as to 
enable the mind to discriminate between them, 
and refer thein to their respective origins in dif- 


ferent parts of the surfaoe. It is also expedaent 
that the internal organs of the body should h»ve 
some sensibility ; but it is better that this should 
be very limited in degree, since the occasmis 
are ieur in which its exercise would be tisefiil^ 
and many in which it would be positively hga* 
rious : hence the nerves €ff sensation are distri-* 
buted in less abundance to these o^ans. 

It is not sufficient that the nerves of touch 
should communicate theperceptions jof the sim]Je 
pressure or resistance of the bodies in contact 
with the iskin : they diould.also. fumidi indica-» 
ti<»)s of other qualities in those bodies, of which 
it is important that the mind be apprized.; such; 
foi^ example, as warmth; or coldness. Whether 
tihese different kinds xrf impressions are all com^ 
veyed by the same nervous fibres it is difficulty 
and perhaps inipossible to determine. 

When these nerves are acted upon ia a way 
which threatens to be n|urious to the. part ini« 
pressed, or to the system at large, it is also their 
province to give warning of the impendii^ evil, 
anii^ to rouse the animal to such exertions as may 
avert it ; and this is effected by the sensation of 
pain, which the nerves are commisffloned to 
excite on all these occasions. Th^ act the part 
of sentitiels, placed at the outposts, to give sigf 
nals of alarm on the a[^ioach ai danger. 

Sensibility to pain must then enter as a ne^ 
cessary constitueot ainong the animal functions; 


for had this property been omitted, the animal 
system would hare been but of short duration, * 
exposed, as it must necessarily be, to perpetual 
casualties of every kind. Lest any imputation 
sdiould be attempted to be thrown on the bene- 
volent intentions of the great Author and De- 
signer of this beautifiil and wondrous fabric, s6 
expressly formed for varied and prolonged euT 
joy men t, it should always be borne in mind that 
the occasional suffering, to which an animal is 
subjected from this law of its organization, is. far 
more than counterbalanced by the consequences 
arising ficom thecapacities forpleasure, with which 
it has been beneficently ordained that the healthy 
exercise of the functions shall be accompanied. 
Enjoyment appears universally to be the main 
end, the rule, the ordinary and natural condi- 
tion : while pain is but the casualty, the excepr 
tion, the necessary remedy, which is ever tending 
to a remoter good, in subwdination to a higher 
law of creation. 

It is a wise and bountifiil provision of i^ture 
that each of the internal parts of the bodx|^as 
been endowed with a particular sensibility to 
those impressions which, in the ordinary course, 
have a tendency to injure its structure ; while it 
has at the same time been rendered, nearly, if 
not completely, insensible to those which are not 
injurious, or to which it is not likely to be ex; 
posed. Tendons and ligaments, for example, 



are insensible to many causes of 
irritation, such as cutting, pricking, and eren 
burning: but the moment they are violently 
stretched, that being the mode in which they 
are most liable to be injured, they instantly 
communicate a feeling of acute pain. The 
bones, in like manner, scarcely ever comnuuii* 
cate pain in the healthy state, except from the 
application of a mechanical force which tends 
to fracture them. 

The system of nerves, compristng those which 
are designed to convey the impressions of touch, 
is universally present in all classes of animals ; 
and among the lowest orders, they appear to con- 
stitute the sole medium of comnranication with 
tiie external world. As we rise in the scale of 
animals we find the faculties of perception ex- 
tending to a wider range, and many qualities, 
depending on the chemical action of bodies, are 
rendered sensible, more ei^ecially those which 
belong to the substances employed as food. 
Hence arises the sense of taste, which may be 
regarded as a new and more refined species of 
touch. This difference in the nature of the mk- 
pressions to be conveyed, renders it necessary 
that the structure of the nerves, or at least of 
those parts of the nerves which are to receive 
the impresfflon, should be modified and adapted 
to this particular mode of action. 


As the sphere of perception is enlarged, it is 
made to comprehend, not merely those objects 
which are actually in contact with the body, but 
also those which are at a distance, and of the 
existence and properties of which it is hi^ly 
important that the animal, of whose sensitive 
faculties we are examining the successive en- 
dowment, should be apprized. It is more espe- 
cially necessary that he should acquire an accu- 
rate knowledge of the distances, situatioi» and 
motions of surrounding objects. Nature has 
accordingly provided suitable organizations for 
vision, for hearing, and fw the percq>tion of 
odours; all of which senses establish extensive 
relations between him and the external world, 
and give him the command of various objects 
which are necessary to supply his wants* pr 
procure him gratification; and which also ap- 
prize him of danger while it is yet remote, 
and may be avoided. Endowed with the power 
of combining all these perceptions, he com- 
mences his career of sensitive and intellectual 
existence; and though he soon learns that he 
is dependent for most of his sensations on the 
changes which take place in the external 
world, he is also conscious of an internal 
power, which gives him some kind of con- 
trol or& many of those changes, and that he 
moves his limbs by his own voluntary act; 


movements which originally, and of themselTes, 
appear, in most animals, to be productive of 
great enjoyment. 

To a person unused to reflection, the pheno* 
mena of sensation and perception may appear to 
require no elaborate investigation. That he 
may behold external objects, nothing more seems 
necessary than directing his eyes towards them. 
He feels as if the sight of those objects were a 
necessaiy consequence of the motion of his eye- 
balls, and he dreams not that there can be any 
thing marvellous in the function of the eye, or 
that any other organ is concerned in this simple 
act of vision. If he wishes to ascertain the 
solidity of an object within his reach, he knows 
that he has but to stretch forth his hand, and to 
feel in what d^ree it resists the pressure he 
gives to it. No exertion even of this kind is 
required for bearing the voices of his companions, 
or being apprized, by the increasing loudness of 
the sound of falling waters, as he advances in a 
particular direction, that he is coming nearer 
and nearer to the cataract. Yet how much is 
really implied in all these apparently simple 
phenomena ! Science has taught us that these 
perceptions of external objects, far from being 
direct or intuitive, are only the final results of a 
long series of operations, produced by agents of 
a most subtle nature, which act by curious and 
complicated laws, upon a refined oi^anization, 


disposed in particular situations in our bodies, 
and adjusted with admirable art to receive their 
impressions, to modify and combine them in a 
eertain order, and to convey them in regular 
succession, and without confusion, to the imme* 
diate seat of sensation. 

- Yet this process, complicated as it may ap- 
pear, constitutes but the first stage of the entire 
function of perception : for ere the mind can 
arrive at a distinct knowledge of the presence 
and peculiar qualities of the extema[l object 
which gives rise to the sensation, a long series of 
mental changes must intervene, and many intel- 
lectual operations must be performed. All these 
take place in such rapid succession, that even 
when we include the movement of the limb, 
Ivhich is consequent upon the perception, and 
which we naturally consider as part of the same 
continuous action, the whole appears to occupy 
but a single instant. Upon a careful analysis of 
the phenomena, however, as I shall afterwards 
attempt to show, we find that no less than twelve 
ilistinguishable kinds of changes, or rather pro- 
cesses, some of which imply many changes, must 
always intervene, in regular succession, between 
the action of the external object on the organ of 
sense, and the voluntary movement of the limb 
which it excites. 

. The external agents, which are capable of 
affecting the different parts of the nervous sys- 

374 THE SESaOBLU* ffTKcnoxs. 

io as to ^odnce w m a t i o n ^ aie cf difloent 
kiadh, and aie gorenied by bvs peroKar to 
thc omslyc s. The utractme of the oigaiiB 
aocordm^y, be adapted^ m each paiticnlaa 
to lecenre the impreasuos auide by theae ageatB, 
and most be modified in exact coofimiuly with 
the physical lavs they obey. Thos the sCnic- 
tue oi that portion of the nerYons ^^sleai 
which recetres Tisoal impreasions, and which k 
tenvd the rafno, nuntbe adapted to theaotioal 
of light ; and the eye, tfaronf^h which the mya 
are made to pass befiire reaching the letma, 
most be constructed with strict reference to the 
laws €f optica. The ear most, in like manner, 
be formed to receiye ddicate impressions finan 
those Tibrations of the air which occasion sonnd* 
The extremities of the nenres, in these and other 
organs of the soises^ are qpread oot into a deb* 
cate expansion of siir£u:e, having a soA» and 
more iuii£wm texture than the rest of the nsrve^ 
whereby they acquire a susceptibility of being 
affected by their own af^vopriale agaits^ and 
by no other. The function of each nenre of 
sense is determinate, and can be executed by no 
other part of the nerrous system. These iunc* 
tions are not interchangeable, as is the case with 
many others in the animal system. No nerra^ 
but the optic nenre, and no port of that nerve^ 
except the retina, is capable, howerer impressed, 
of giying rise to the sensation of light : no part 


ef the nenraus syst^u, but the auditory nerve 
ean convey that of sound ; and so of the rest. 
T%e creduHty of the public has sometimes been 
imposed upon by persons who pretended to see 
by means of their fingers : thus^ at Liverpool, 
the celebrated Miss M'Avoy contrived for a 
long time to persuade a great number of perscms 
that she really possessed this miraculous pow^r. 
Equally unworthy of credit are all the stories of 
persons, under the influence of animal magnet- 
ism, hearing sounds addressed to the pit of the 
stomach, and reading the pages of a book ap- 
plied to the skin over that oi^an. 

In almost every case the impression made 
npon the sentient extremity of the nerve which 
is appropriated to sensation, is not the direct 
efibet of the external body, but results fiom the 
agency of some intervening medium. There is 
always a portion irf the organ of sense interposed 
between the object and the nerve on which the 
impression is to be made* The object is never 
allowed to come into direct contact with the 
nerves; not even in the case of touch, where 
the oigan is defended by the cuticle, through 
which the impression is made, and by which 
diat impression is modified so as to produce the 
proper effect on the subjacent nerves. This ob- 
servation applies with equal force to the organs 
of taste and of smell, the nerves of which are 
not only sheathed with cuticle, but defended 


fimn too YioleDt an actioQ by a secretioD ex- 
pfesdy pioTided for thai purpose. In the seoMB 
of hesffing and of Tiflkm^ the changes which take 
place in the organs interposed between the ex- 
ternal impresBiiMis and the nerres, are still more 
lemarkable and important, and will be respec- 
tiydy the solgects of s^mrate inquiries. The 
objects of these senses, as wdl as those of smdl, 
being situated at a distance, produce their first 
impressHMis by the aid of some me&um, exterior 
to oar bodies, through which their infln^Mse 
extends: thus, the air is the usual medinm 
through which both li^t and sound are con* 
yeyed to our organs. Hence, in oder to under- 
stand the whole series of phencmiena belonging 
to sensation, regard must be had to the physical 
laws which regulate the transmission ci these 
agents. We are now to consider these inter- 
processes in the case of each of the 


Chapter II . 


I HAVE already had occasion to point out the 
structure of the integuments, considered in their 
inechamcal office of protecting the general frame 
of the body * ; but we are now to view them in 
their relation to the sense of touch, of which 
they are the immediate organ. It will be recol- 
lected that the carium forms the principal portion 
of the skin ; that the cuticle composes the outer- 
most layer ; and that between these there occurs 
a thin layer of a substance, termed the rete mu- 
easmm. The corium is constructed of an inter- 
texture of dense and tough fibres, through which 
a multitude of blood vessels and nerves are 
interspersed; but its external surface is more 
vascular than any other part, exhibiting a fine 
and delicate net-work of vessels, and it is this 
portion of the skin, termed by anatomists the 
vascular plexus^ which is the most acutely sen- 
sible in every point : hence, we may infer that 
it> contains the terminations of all the nervous 
filaments distributed to this organ, and which 

• Volume i, p. 112. 


are here foun^ to divide to an extreme degree 
of minuteness. 

When examined with the microscope, this ex- 
ternal surface presents a great number of minute 
projecting filaments. Malpigfai first discoveied 
this structure in the loot of a pig ; and gave 
these prominences the name of papilla. It is 
probable that each of these papillae contains a 
separate branch of the nerves of touch, the ulti« 
mate ramifications of which are spread over the 
surface ; so that we may consid^* these papilhe, 
of which the ass^nblage has been termed the 
corpus papiUare^ as the principal and immediate 
organ of touch. This structure is particulariy 
conspicuous on those parts oi the skin which are 
more especially appropriated to this sense, such as 
the tips of the fingers, the tongue, and the Iqps : 
in other parts of the surface, which are endowed 
with less sensibility, the papill» are scarcely 
visible, even with the aid of the microscope. 

The surface of the cerium is exquisitely sen-* 
sible to all irritations, whether proceeding from 
the contact of foreign bodies, ot from the imr 
pression of atmospheric air. This extreme sen* 
sibility of the cerium would be a source of con- 
stant torment, were it not defended by the cutidey 
which is unprovided with either blood-vessels oe 
nerves, and is, therefore, wholly insensible. Fov 
the same reason, also, it is litde liable to change^ 
and is thus, in both respects, admirably calcu- 

TOUCH. 379 

lated to afibrd protection to the finely oiganized 

Although the cuticle exhibitB no traces of vas- 
cularity, it is by no means to be regarded as a 
dead or inorganic substance, like the shells of 
the m<dlusca. That it is still part of the living 
system is proved by the changes it frequently 
undergoes, both in the natural and the diseased 
conditions of the body. It is perpetually, though 
slowly, undergoing decay and renovation; its 
external surface drying off in minute scales, 
and in some animals peeling off in lai^ por- 
tions. When any part of the human skin is 
scraped with a knife, a grey dust is detached 
from it, which is found to consist of minute 

By repeated friction, or pressure of any part 
of the skin, the cuticle soon acquires an increase 
of thickness and of hardness ; this is observable 
in the soles of the feet, and palms of the hands, 
and in the fingers of those who make much use 
of them in laborious work. But this greater 
tfakkness in the parts designed by nature to 
suffer considerable pressure, is not entirely the 
effect of education ; for the cuticle, which exists 
before birth, is found even then to be much 
thicker on the soles of the feet, and palms of the 
hands, than on other parts. This example of 
I^ovident care in originally adjusting the struc- 
tures of parts to the circumstances in which 


they are to be placed ai an after period, would 
of iteelf, were it a solitary instance, be well fitted 
to can forth our admiration. Bot the proo6 of 
design in the adaptation of organs to their res- 
pectiye purposes mnlti|dy upon ns in such pro- 
fusion, as we study in detail each department of 
the animal economy, that we are apt to oYoiook 
individual instances, unless they are particulaiiy 
brought before our notice. How often have we 
witnessed and profited by the rapid renewal of 
the cuticle, when by any accident it has been 
destroyed, without adverting to the nature of the 
process which it implies; or reflected that the 
vessels of the skin must, on all these occasions, 
supply the materials, out of which the new 
cuticle is to be formed, must effect their, com- 
bination in the requisite prop<»tions, and must 
deposit them in the precise situations in which 
they are wanted ! 

. Different animals present remarkable differ- 
ences in the thickness and texture of the cuticle, 
according to the element they are destined to 
inhabit, and the situations in which they are 
most frequently placed. Provision is in many 
cases made for preserving the cuticle from the 
injury it would receive from the long continued 
action of the air or water ; for it is apt to become 
rigid, and to peel off, from exposure to a very 
dry atmosphere ; and the constant action of 
water, on the contrary, renders it too soft and 
spongy. In order to guard against both these 

TOUCH. 381 

effects, the skin has been furnished, in various 
parts of its surface, with a secreting apparatus, 
which poiirs out unctuous or mucilaginous fluids : 
the oily secretions being more particularly em- 
ployed as a defence against the action of the 
air, and the mucilaginous fluids as a protection 
against that of water. 

The conditions on which the perfection of the 
sense of touch depends are, first, an abundant 
provision of soft papillae supplied with numerous 
nerves; secondly, a certain degree of fineness 
in the cuticle ; thirdly, a soft cushion of cellular 
fflibstance beneath the skin ; fourthly, a hard 
resisting basis, such as that which is provided in 
the nails of the humian fingers ; and lastly, it is 
requisite that the organ be so cohstnicted as to 
be capable of being readily applied, in a variety 
of directions, to the unequal surfaces of bodies ; 
for the closer the contact, the more accurate will 
be the perceptions conveyed. In forming an 
estimate of the degree of perfection in which 
this sense is exercised in any particular animal, 
we- must, accordingly, take into account the 
mobility, the capability of flexion, and the figure 
of the parts employed as organs of touch. 

As touch is the most important of all the 
senses, inasmuch as it is the foundation of all 
our l^nowledge of the material world, so its rela- 
tive degrees of perfection establish marked dif- 
ferences in the intellectual sagacity of the several 
tribes, and have a considerable influence on the 


assignment of their proper station in the scale of 

Although the power of receiving obscure im- 
pressions from the contact of external bodies, 
and of perceiving variations of temperature, is 
probably possessed by all animals, a small num- 
ber only are provided with organs specially 
appropriated for conveying the more delicate 
sensations of touch. The greater part of the 
surface of the body in the testaceous MoUusca is 
protected by a hard and insensible covering of 
shell. The integuments of Insects, especially 
those of the Coleoptera, are in general too rigM 
to receive any fine impressions from the bodicB 
which may come in contact with them ; and the 
same observation applies, with even greater force, 
to the Crustacea. The scales of Fishes, and of 
Reptiles, the solid encasements of the Chelonia, 
the plumage of Birds, the dense coating of the 
Armadillo, the thick hides of the Rhinoceros, 
and other Pachydermata, are evidently incom*- 
patible with any delicacy of touch. This nicer 
faculty of discrimination can be enjoyed only 
by animals having a soft and flexible integu- 
ment, such as all the naked Zoophytes, Worms, 
and MoUusca, among the lower orders, and Ser- 
pents, among the higher. The flexibility of the 
body or limbs is another condition which is ex*- 
tremely necessary towards procuring extensive 
and correct notions of the relative positions of 
external objects. It is essential therefore that 

TOUCH. 383 

those instruments which are more partici;dariy 
intended as organs of touch, should possess this 

It will not be necessary to enter into a minute 
description of these organs, because they have, 
for the most part, been already noticed as in- 
struments of prehension ; for the sense of touch 
18 in general exercised more particularly by the 
same parts which perform this latter function* 
Thus the tentacula of the various tribes of 
Polypi, of Actiniae, and of Annelida, are organs 
both of prehension and of touch. The tubular 
feet of the Asterias and Echinus are, in like 
maimer, subservient both to the sense of touch, 
and to the faculty of progressive motion. The 
feet of Insects and of Crustacea are well cal- 
culated, indeed, by their jointed structure, for 
being applied to the surfaces, and to different 
sides of bodies ; but they are scarcely ever em- 
ployed in this capacity ; being superseded by the 
palpi, which are situated near the mouth. When 
insects are walking, the palpi are incessantly 
applied to the surface on which they advance, 
as if these organs were especially employed to 
feel their way. There can be little doubt, how- 
ever, that, in most insects, the principal organs 
of touch are the Antenna, also denominated, from 
their supposed office, the feelers.* 

Some idea of the great variety in the forms of 

* The German name for them, fuhlhorner^ or the feeling 
horns, is founded on the same notion. 



the antenofe of insects may be obtaiaed from 
the specimens delineated in Fig. 381, 'wbtcfa 
shows a few of the most remarkable.* 

* In this figure, A represents the rorm of anteante, technicall j 
denominated Antenna capitulo uncinate, u exemplified in the 

6. is the A. plloso-verticillata, as in the PMychoda ocellarit. 

C. .A. biclavata, (Claviger langieomia). 

D. .A. triangularis, (LopHotia). 

E. .A. clttvata, (Masarit). 

F. .A. capit. lamelbto, (Melotontka mas). 

G. .A. capit. fissile, (Apkodiusfossor). 
H . . A. fuBironnia, (Zygana). 

1.;A. capitata, (Atcaliphus). 

K. . A. furcata, (NepaJ. 

L. .A. bipectinata, (Bombyi). 

M. . A. irregularis, ( Agaon paradoxtim). 

N. . A. cordata, (Diapeiis boleti). 

O..A. bipectinata, (Ctenophora). 

P.. A. palmata, (Nepa cinerea). 

Q, . A. ensiformis, ( Truxalis). 

R. .A. letacee, (Cerambyx). 

TOUCH. - 385 

Thie universality of these organs among every 
species of this extensive class of animals^ their 
great flexibility, arising from their jointed stnic-* 
ture/ their incessant motion when the insect 
is walking/ and their constant employment in 
examining the surfaces of all the bodies with 
which they come in contact, sufficiently point 
them out as instruments of a very delicate, sense 
of touch. Organs of this kind were particularly 
necessary to insects, since the horny nature of 
the integuments of the ^eater number pre* 
eludes them from imparting any accurate per-* 
eeptions of touch* 

It has been conjectured that the ahtennsa of 
insects are the organs of other senses besides 
that of touch. If an insect be deprived of itd 
antennae, it eitiier remains motionless, or if it 
attempt to walk or fly, appears bewildered, and 
moves without aiiy apparent object. Huber 
found that bees are enabled, by feeling with 
their antennae, to execute their various works in 
the interior of the hive, where, of course, they 
can have no assistance from light. They empldy 

* The number of segmenU into which these organs are divided 
is often very great. In the Gryllotalpa, or mole cricket, it 
amounts to above 100. (Kidd, Phil. Trans, for 1825, p. 211.) 
This insect has, besides the antennee on the head, two posterior 
or caudal antennae, which are not jointed, excepting at their 
very commencement. These are extremely sensible, and serve 
|>robably to give the animal notice of the approach of any 
annoyance from behind, lb. p. 216. 



thiese Cleans perpetually while building Hxe 
comb8» pouring honey into the magaaanes, ascer-' 
taining the presence of the queen, and feeding 
and tending, the larvee. The same naturalist 
observes, also, that it is principally by means of 
the antennae that : these social insects coinmnni- 
cate to one another their impressions and their 

The different modes in which ants, when they 
happen to meet during their excursions, mu- 
tually touch one another with their antenne, 
appears to constitute a kind of natural lan- 
guage understood by the whole tribe. ThiA> 
contact of the! antennae evidently admits of a 
great variety of modifications, and seems capable 
of supplying all the kinds of information which 
th^e insects have occasion to impart. It would 
seem impossible, indeed, for all the individuals: 
composing these extensive societies to! co-operate 
effectually in the execution o£ many works, 
calculated for the general benefit of the com- 
munity, unless some such means of commu- 
nication existed. There is no evidence that 

Mm * 

sound is the medium of this intercourse ; for 
none, audible to us at least, was ever known, 
to be emitted by these insects. Their mode 
of conversing together appears to be simply 
by touching one another in different ways with 
the antennae. Ruber's observations on thi^ 

TOUCH. 387 

subject are exceedingly curious.* He remarks 
that the signal denoting the apprehension of 
danger, is made by the ant striking its head 
against the corselet of every ant which it chances 
to meet. Each ant, on receiving this intima- 
tion, immediately sets about repeating the 
same signal to thei next ant which comes in its 
way ; and the alarm is thus disseminated with 
astoniffhing rapidity throughout the whole so- 
ciety. Sentinels are at all times stationed on 
the outside of the nests, for the purpose of 
apprizing the inhabitants of any danger that 
may be at hand. On the attack of an enemy, 
these guardians quickly enter into the nest, and 
spread the intelligence on every side : the whole 
swarm is soon in motion, and while the greater 
number of ants rush forwards with desperate 
fiiry to repel the attack, others who are entrusted 
with the office of guarding the eggs and the 
larvae, hasten to remove their charge to places of 
greater security. 

When the queen bee is forcibly taken away 
from the hive, the bees which are near her at 
the time, do not soon appear sensible of her ab- 
sence, and the labours of the hive are carried on 
as usual. It is seldom before the lapse of an 
hour, that the working-bees begin to manifest 
any symptoms of uneasiness: they are then 

* See his '' Recherches sur lea moeurs des fourmis indigenes/' 


observed to quit the larvae which they had been 
feeding, and to run about in great agitation, to 
and fro/ near the cell which the queen: had oc- 
cupied before her abduction. They then move 
over a wider circle, and on meeting with, such of 
their companions as are not aware of the disaster, 
communicate the intelligence by crossing their 
antennae and striking lightly with them. ;The 
bees which receive the news become in their 
turn agitated, and conveying this feeling where-, 
ever they go, the alarm is soon participated by 
all the inhabitants of the hive. All rush forwards^ 
with tumultuous precipitation, eagerly seeking 
their lost queen ; but after continuing the search 
for some hours, and finding it to be fruitless, 
they appear resigned to their misfortune ; the 
noisy hubbub subsides, and the bees quietly 
resume their labours. 

A bee, deprived of its antennae, immediately 
becomes dull and listless : it desists from its 
usual labours, remains at the bottom of the hive, 
seems attracted only by the light, and takes the 
first opportunity of quitting the hive, never more 
to return. A queen bee, thus mutilated, ran 
about, without apparent object, as if in a state, 
of delirium, and was incapable of directing, her 
trunk with precision to the food which was 
offered to her. Latreille relates that, having de- 
prived some labouring ants of their antennae, lie 
replaced them near the nest ; but they wandered 

TOUCH. 389 

in all directions, as if bewildered, and uncon* 
scions of what they were doing. Some of their 
companions were seen to notice their distress; 
and approaching them with apparent compas* 
sion, applied their tongues to the wounds of the 
su£(erers, and anointed them with their saliva. 
This trait of sensibility was repeatedly witnessed 
by Latreille, while watching their moYements 
with a magnifying glass. 

The Arachnida, from the mobility of their 
limbs, and the thinness of their cutaneous invest- 
ment, have a very delicate sense of touch ;. 
Among the Mollusca, it is only the higher orders 
of Cephalopoda that enjoy this sense in any con- 
siderable degree, and they are enabled to exer- 
cise it by means of their long and flexible ten- 
taenia. Many bivalve mollusca have, indeed, 
a set of tentacula placed near the mouth, but 
they are short, and of little power. It is pro- 
bable that the foot may also be employed by 
these animals as an organ of touch. 

Fishes are, in general, very ill-constructed for 
the exercise of this sense, and their fins are used 
for no other purposes than those of progressive 
motion. That part of the surface which pos- 
sesses the most acute feeling is the under-side, 
where the integuments are the thinnest. The 
chief seat of the sense of touch, however, is the 
lip, or end of the snout, which is largely sup- 
plied with nerves; and perhaps the cirr/tiy or 


little yermiform processes called ifarbeh, which 
in some species are appended to the mouthy 
may be subservient to this sense.* These ym-' 
cesses in the Silurus glanis are moved by par- 
ticular muscles. 

Serpents, from the great flexibility <^ their 
spine^ are capable oC grasping and twining round 
objects of almost any shape^ and of taking, as 
it were, their exact measure. This conformation 
must be exceedingly favourable to the acquisi- 
tion of correct perceptions . of touph. As it is 
these perceptions, which, as we sj^all afterwards 
find, lay the foundation of the most perfect ac- 
quaintance with the tangible properties of sur* 
rounding bodies, we may presume that this 
power contributes much to the sagacity possessed 
by these animals. It has been said of Serpents, 
that their whole body is a hand, conferring some 
of the advantages of that instrumeat. Hellman 
has shown that the slender bifurcated tongu0 
of these animals is used for the purposes of 
touch, t 

In those species of Lizards which are ena- 
bled by the structure of their feet to clasp the 
branches of trees, as the Gecko and the Chat 

* These kind of tentacula are remarkable for their length and 
mobility in the Lophins piscatoriuSf or Angler ; and it is said 
that they are employed by the fish, while lurking in ambush, as 
a decoy to other fishes, which they entice by their resemblance to 

t Quoted by Blumenbach. 

TOUCH. sot 

meKon, and whose tails also are prehensile, 
we inust, for the same reason, presume that the 
sense of touch exists in a more considerable 
degi^ than in other saurian reptiles, which do 
not possess this advantage. Th^ toes of Bisds 
are also well calculated to perforin the office of 
organs of touch, from the number of their arti^ 
eolations and their . diyergeut position, and from 
the papillsei vHlth which their skin abounds,, acf^ 
companied as they are with a laiige supply of 
nerves. Those birds, which, like the Parrot, 
employ the feet as organs of piiehension, probably 
enjoy a greater developement of this sense* The 
skin which covers the bills of aquatic birds is 
supplied by very large nerves, and consequently 
possesses great sensibility. This structure enables 
them to find their food, which is concealed in the 
mud, by the exercise of the seixse of touch 
residing in that organ, A similar structure, 
probably serving a similar purpose, is found in 
the Ornithorhyncus. 

Among Mammalia, we find the seat of this 
sense firequently transferred to the lips, and ex^ 
txemity of the nostrils, and many have the nose 
{urolonged and flexible^ apparently with this 
view. This is the case with the Shrew and the 
Mole, which are burrowing animals, and still 
more remarkably with the Pachydermata, where 
this greater sensibility of the parts about the 
face seems to have been bestowed as some com*' 


pensatibn for the general obtaseness of fec^nip 
resulting from the thicknesd of the hide which 
covers the rest of the body. Thns the ilhi** 
noceros has a soft, hook-shaped extension of 
the upper lip, which is always kept moist, 
in order to preserve its sensibility as an organ 
of touch. The Hog has the end of the nose 
also constructed for feeling; though it is not so 
well calculated fw distinguishing the form of 
objects, as where the oi^an is prolonged in the 
form of a snout, which it is in the Tapir, and in 
a still higher degree in the admirably constructed 
proboscis of the Elephant, which as an organs 
both of prehension and of touch, forms the 
pearest approach to the perfect structure of the 
human hand. 

The lion, Tiger, Cat, and other animals oi the 
genus FeliSy have whiskers, endowed at their 
roots with a particular sensibility, from being 
largely supplied with nerves. The same is the 
case with the whiskers of the Seal. 

The prehensile tails of the American monkeys 
are doubtless fitted to convey accurate percept 
tions of touch, as well as the feet and hand&; 
as may be inferred from the great size of the 
nervous papillee, and the thinness of the cuticle 
of those parts. 

The sense of touch attains its greatest degree 
of excellence in the human hand, in which it is 
associated with the most perfect of all instru-^ 

TASTE. 393 


of prehension. But as the structure and 
lonGtions of this organ are the exclusiye sul^ects 
of another of these treatises, I shall refrain from 
any farther remarks respecting them. 

Chapter III. 



The senses of taste and smell are intended to 
convey impressions resulting from the chemical 
qualities of bodies, the one in the fluid, the other 
in the gaseous state.* There is a considerable 
analogy between the sensations derived from 
these two senses. The organ of taste is the 
surface of the tongue, the skin of which is fur- 
nished with a large proportion of blood-vessels 
and nerves. The vascular plexus immediately 
covering the corium is here very visible, and 
forms a distinct layer, through which a great 
tiumber of papillee pass, and project from the 

surface, covered with a thin cuticle, like the pile 

• • • 

' ^ Bellini contended that the different flavors of saline bodies 
ivere owing to the peculiar figures of their crystalline particles. 
Jt is strange that Dumas should have thought it worth while 
seriously to combat this extravagant hypothesis, by a laboured 


of Telvet. In the fore part of the. hitman UM^ne 
these papilke sxe viable eren to the naked eye, 
and especially in certain morbid oonditioiis of 
the organ.? They are of diffcrmit kinds^ hut 
it is only those which are of a conical shape 
that are the seat of taste. If these papillee be 
touched with a fluid, which has a strong taste, 
such as vinegar, applied by means of a camel- 
hair pencil, they will be seen to become elon- 
gated by the action of the stimulus, an effect 
which probably always accompanies the percep- 
tion of taste. 

The primary use of this sense, the organ of 
which is placed at the entrance of the alimen- 
tary canal, is evidently to guide animals in the 
choice of their food, and to warn them of the 
introduction of a noxious substance into the 
stomach « With respect to the human species^ 
this use has been, in the* present state of society, 
superseded by many acquired tastes, which have 
supplanted those originally given to us by na- 
ture : but in the inferior animals It still retains 
its primitive o£Gice, and is a sense of great im- 
portance to the safety and welfare of the indivi- 

* This is particularly the case in scarlatina, in the early stage 
of which disease they are eloag^ated, and become of a bright red 
colour, from their minute blood-vessels being distended with 
blood. As the fever subsides the points of the papills collapse* 
and acquire a brown hue, giving rise to the appearance known 
by the name of the strawberry tongue. 

TASTE. 395 

dual, from its operation being coincident with 
those of natural instincts. If; as it is said, thes^ 
instincts are still met with among men in a 
savage state» they are soon weakened or effaced 
by ciyilization. 

The tongue, in all the inferior classes Qf vei;^ 
tebrated animals, namely Fishes, Reptiles, and 
Birds, is scarcely ever constructed with a view 
to the reception of delicate impressions of taste ; 
being generally covered with a thick, and often 
homy cuticle ; and being, besides, scarcely ever 
employed in mastication. This is the case, also, 
with a large proportion of quadrupeds, which 
swallow their food entire, and which cannot, 
therefore, be supposed to have the sense of taste 
much developed. 

Insects which are provided with a tongue or 
a proboscis may be conceived to exercise the 
sense of taste by means of these organs. But 
many insects possess, besides these, a pair of 
short feelers, placed behind the true antennae; 
and it has been observed that, while the insect 
is taking food, these organs are in incessant mo-r 
tion, and are continually employed in touching 
and examining the food, before it is introduced 
into the mouth : hence, some entomologists have 
concluded that they are organs of taste. But it 
must be obvious that in this, as in every other 
instance in which our researches extend to 
beings of such minute dimensions, and which 


occupy a station, in the order of sensitive ex- 
istence, so remote from ourselves, we are wander- 
ing into regions where the only light that is 
afforded us must be borrowed from vague and 
fanciful analogies, or created by the force of a 
Tivid and deceptive imagination. 

Chapter IV. 


Animal life being equally dependent upon the 
salubrious qualities of the air respired, as of 
the food received, a sense has been provided- 
for discriminating the nature of the former, 
as well as of the latter. As the organs of taste 
are placed at the entrance of the alimentary 
canal, so those of smell usually occupy the be- 
ginning of the passages for respiration, where 
a distinct nerve, named the olfactof^^ appro- 
priated to this office, is distributed. 

The sense of smell is generally of greater 
importance to the lower animals than that of 
taste ; and the sphere of its perceptions is in 
them vastly more extended than in man. The 
agents, which give rise to the sensations of 


smell, are cert&in effluvia, or particles of ex^ 
treme tenuity, which are disseminated yery 
quickly through a great extetlt of atmospheric 
air. It is exceedingly, difficult to conceivehow 
matter so extremely rare and subtle as that 
which composed these odorous effluvia can re- 
tain the power of producing any sensible im- 
pression on the animal organs : for its tenuity is 
so extraordinary as to exceed all human com- 
prehension. The most copious exhalations front 
a variety of odoriferous substances, such as miisk^ 
valerian, or assafoetida, will be continually 
emanating for years, without any perceptible loss 
of weight in the body which supplies them. It 
is well known that if a small quantity of n^usk 
be enclosed for a few hours in a gold box^ and 
then taken out, and the box cleaned as carefully 
as possible with soap and water, that box will 
retain the odour of musk for many years ; and 
yet the nicest balance will not show the siqiallest 
increase of its weight from this impregnation* 
No facts in natural philosophy afford more 
striking illustrations of the astonishing, wnd 
indeed inconceivable divisibility of matter, than 
those relating to odorous effluvia. 

It would appear that most animal and vege- 
table bodies are continually emitting these subtle 
effluvia, of which our own organs are not suffi*- 
ciently delicate to apprize us,* unless when they 


are much concentrated, but which are readily 
perceiTcd and distinguished by the lower ani- 
mals ; as may be inferred from their actions. A 
dog is known to follow its master by the scefnt 
alone, through the avenues and turnings of a 
crowded city, accurately distinguishing his track 
amidst thousands of others. 

The utility of the sense of smell is not con- 
fined to that of being a check upon the respira- 
tion of noxious gases ; for it is also a powerful 
auxiliary to the sense of taste, which of itself, 
and without the aid of smell, would be very 
vague in its indications and limited in its range. 
What may have been its extent and delicacy in 
man, while he existed in a savage state, we have 
scarcely any means of determining ; but in the 

* present artificial condition of the race, resulting' 
from civilization and the habitual cultivation of 
other sources of knowledge, there is less neces^* 
sity for attending to its perceptions, and our sen- 
sibility to odours may perhaps have diminished 
in the same proportion. It is asserted both 
by Soemmerring and Blumeiibach that the organ 

- of smell is smalter in Europeans, and other civi^ 
lized races of mankind, than in those nations of 
Africa or America, which are but little removed 
firom a savage state : it is certainly much less 
developed in man than in most quadrupeds. To 
the carnivorous tribes, especially, it is highly 

SMSLL* 399 


useful in enabling them to discover their natural 
food at great distances. 

The cavity of the nostrils, in all terrestrial 
vertebrated animals is divided into two by a 
vertical partition ; and the whole of its internal 
surface is lined by a soft membrane, called the 
Schnetderian memhrane^^ which is constantly 
kept moist, is supplied with numerous blood- 
vessels, and upon which are spread the ultimate 
ramifications of the olfactory nerves. The rela- 
tive magnitude of these is much greater 
in carnivorous quadrupeds than in those which 
subsist on vegetable food. In quadrupeds, as 
well as in man, these nerves are not collected 
into a single trunk in their course towards the 
brain, but compose a great number of ^laments^ 
which pass separately through minute perfora- 
tions in a plate of bone, (called the ethmoid bane) 
befbre they enter into the cavity of the skull, 
and join that part of the cerebral substance with 
which they are ultimately connected. 

The surface of the membrane which receives 
the impressions from odorous effluvia, is con- 
siderably increased by several thin plates of 
bone, which project into the cavity of the nos- 
trils, and are called the turbinated bones. These 
are delineated at t, t^ in F.ig^ 382, as they appear 

* It has been so named in honour of Schneider, the first ana- 
tomist who gave an accurate description of this membrane. 


in a vertical and longitudinal section of the 
carity of the human nostril, where they are seen 

covered by the Schneiderian memhrane.* A 
transvCTse and vertical section of these parts is 
given in Fig. 383.t The turbinated bones are 
curiously folded, and often convoluted in a spiral 
form, with the evident design of obtaining as 

' This figure shows the branches of the olfactory nerre (o), 
passing through the thin cribrifom plate of the ethmoid bona, 
and distributed over that membrane. Several of the cdia, 
which open into the cavity, are also seen; such as the large 
sphenaidal sinus (s), the frontal sinus (f), and one of the eth- 
moidal cells (c). R, is the nasal bone ; p, the palate ; and i, 
the mouth of the Eustachian tube, which leads to the ear. 

t In this figure, s, is the septum, or partition of the nostrils^ 
on each side of which are aeea the sections of the turbinated 
bones projecting into the cavity ; the ethmoid cells (c), situated 
between the orbits (o) ; and the Antrum maxUtan (a), which 
is another large cavity communicating with the oostrils. 

great an extent of surface as possible vithin 
the confined space of the Daaal cavity. This 

turbinated, or spiral shape, chiefly characterises 
these bones among herbivorous quadrupeds : 
in the horse, for example, the turbinated 
bones are of a lai^ diameter, and extend the 
whole length of the prolonged nostrils. Their 
structure is exceedingly intricate ; for while 
tb^ retain externally the general shape of an 
oblong spiral shell, they are pierced on all 
^eir internal sides with numerous perforations, 
through which the membrane, t<^ether with the 
fine branches of the nerves, passes freely from 
one side to the other. The cavities resulting 
from the convolutions are intersected by un- 
perfprated partitions of extramdinary tenuity^ 
serving both to support the arches of bone, and 

VOL. II. o D 


to fbrniah a still greater surface for the extenuoa 
of the ol&ctory membrane. In the She^, the 
Goat, and the Deer, the strncture is very similar 
to that just described ; bnt the ccHiTolutions are 
double, with an intermediate paititioo, so as to 
resemble in its transrerae section the capitei <^ 
an Ionic column.* Th^ are shova at (t) in the 
transT»8e section of the nostrils of a sh^p in 
Fig. 384. 

In caroiTOrous quadrupeds -the structure, of 
these bones is still mtve intricate, and is cal? 
culated to afford a far more extensive surfaca 

* In a Bpeciea of Antelope described by Mr. Hodgson, catitief 
exist, situated immediately behind the ordinary nostrils, an J 
conmunicstingr with them, lliese accessory nostrils are conjee* 
tured to be usefal to tbis exceedingly fleet animal by ftcSUtatiiif 
its breatbing, while it is exerting its utmost speed; for tfat 
expanuon of the nostrils opens also these posterior cavities, the 
side* of which, bring ^aadc, remain dilated. Journal of the 
Antic Seoiety, Fek 188«, p. 09. 

fiir the distribution of the olfactory nerve/ In 
the Seal this conformation is most fiilly de^ 
i^loped, and the bony plates are here not tnr^ 
binated, but ramified, as shown at t in Fig. 385^ 
Bi^it or more principal branches arise from the 
main trunk ; and each of these is aAerwardi 
divided and sabdirided to aU' extreme degree of 
minuteness, so as to form in all many hundred 
plates. The nA&ctocy membrane* with aU its 
nerves, is closely applied to every plate in this 
vast ass^nblage, as wdl. as to the main trunks 
and to the internal surface of the snniounding 
cavity : so that its exient^ cannot . be less than 
19D square inches in each nostril. An organ of 
flfoch exquisite sensibility requires an extraor- 
dinary provision for securing, it againsi; injury, 
by the power of voluntarily excluding noxious 
vi^>ours ; and nature has supplied a mechanisro 
for this express purpose,. enaUing the animal to 
dose at j^riieasuce the orifice of the nostril. The 
iiog, which, in its neural state, ienibsists'wboHy 
<m- vegetable feed^ resen^bles herbivorous tribes 
in this external form and rdative magnitude 
of the. turbinated bones; but they are more 
simple in their structure, being fiuined of single, 
and slightly ooavoluted {dates^.witfai^ut partitions 
cr pevfinrations. In this lespect they approach 
to the human structure, wJbichis even. lees com^ 
plicated, and indicates a greater affinity with 
vigetable than with animal feeders. Man, in*- 


deed, distinguishes more accurately vegetable 
odours than those proceeding firom animal sub- 
stances ; while the reverse is observed with re- 
gard to quadrupeds whose habits are decidedly 
carnivorous. A dog, for instance, is regardless of 
the odour of a rose or violet; and probably, as 
he derives from them no pleasmre, is unable to 
discriminate the one from the other. Preda^ 
cious animals, as Sir B. Harwood observes, 
require both larger olfactory nerves, and a more 
extensive surface for their distribution, than the 
vegetable eaters. The food of the latter is ge- 
nerally near at hand ; and as they have occasion 
only to select the wholesome from the noxious 
plants, their olfactory organs are constructed fcur 
the purpose of arresting the effiuvia of odorous 
substances immediately as they arise. The former 
are often under the necessity of discovering the 
lurking places of their prey at a considerable 
distance, and are therefore more sensible to the 
weak impressions of particles widely diflosed 
through the surroundii^ medium, or slightly ad* 
hering to those bodies, with which the object ef 
their pursuit may have come into contact. ' 

The olfactory bones of birds are constructed 
very much on the model of the spiral bones of 
herbivorous quadrupeds, and vary but little iti 
the different species. Fig. 386 exhibits their 
appearance in the Turkey : but the size of the 
olfactory nerves of birds of prey greatly exceeds 

tluit of the same nerves in graiUTorous birds. 
In the latter, indeed, they are exceedingly small ; 
and as the natural food of that tribe has but little 
odour, we find that they are easily deceived by 

any thing which bears a resemblance to it. Sir 
Busick Harwood relates that some poultry, which 
were usually fed with a mixture of barley meal 
and water, were found to have swallowed, by 
mistake, nearly the whole contents of a pot of 
white paint. Two of the fowls died, and two 
others became paralytic. The crops of the 
latter were opened, and considerably more than 
a pound of the poisonous composition taken from 
each ; and the crops, either naturally, or from 
the sedative effects of the paint, appeared to 
have so little sensibility that, after the wounds 
were sewed up, both the fowls eventually reco- 

The olfactory nerves are conspicuous in the 
Duck, both £rom their size andmode of distribu- 


tion. They aoe aeea in Fig. 3ft7, psMUDg out 
throng^ the orbU of the eye (o) in two large 

branches, an upper one (u), and a lower one (l), 
the ramifications of which are spread over the 
mandibles, both within and without. For the 
protection of the highly sensible extremity of 
the beak, against the injurious unpressions of 
hard bodies, a homy process (p), similar, both in 
form and office, to the human nail, is attached to 
it, and its edges guarded by a narrow border d 
the same homy material; these receive a first, 
and fainter impression, and admonish the animal 
of approaching danger ; if none occur, the mat- 
ter is then submitted to the immediate scmtlDy 
of the nerves themselves, and is swallowed an 
rejected according to their indication.* 

It has been generally asserted that Vultures, 
and other birds of prey, are gifted with a highly 
acute sense of smell; and that they can discover 
hy means of it the carcase of a dead uiimal at 
great distances : but it appears to be now suffi- 

* Such is the accotint fpnn hj Sir Baaick Ruvood, !n kn 
" System of CoaparatifB Anatomf and PbTuole^," p.S&- ^ -. 

0MBLL. 407 

dkmtly eslabliriied by the obwnratiotift and each 
periment8 of Mr. Audubon, that these luids in 
zeality poMess the sense of smell in a degree 
very inferior to carnivorous quadrupeds; and 
that so far from guiding them to their prey from 
a distance, it affords them no indication of its 
presence, even when ^lose at hand* The follow- 
ing experiments appear to be perfeetly con*' 
elusive on this subject. Having procwed the 
skin of a deer, Mr. Audubon stuffed it full of 
hay ; and after the whcHe had become perfectly 
dry and hard, he placed it in the middle of an 
open field, laying it down on its back, in the 
attitude of a dead animal. In the course of a 
few minutes afterwards, he observed a vulture 
flying towards it, and alighting near it. Quite un* 
suspicious of the deception, the bird immediately 
proceeded to attack it, as usual, in the most vul- 
nerable points. Failing in his object, he next, 
with much exertion, tore open the seams of the 
skin, where it had been stitched together ^ and 
appeared earnestly intent on getting at the flesh, 
which he expected to find within, and of the 
absence of which, not one of his senses was able 
to inform him. Finding that his efforts, which 
were long reiterated, led to no other result than 
the pulling, out laige quantities of hay, he at 
length, though with evident reluctance, gave up 
the attempt, and took flight in pursuit of other 
game to which he was led by die sight alone. 


and which he was not long in disccnrering aad 

Another experiment, the conTerse of the fiistv 
was next tried. A large dead hog was cracealed 
in a narrow and winding rayine, abont twenty 
feet deeper than the surface of the earth around 
it, and filled with briers and high cane. This 
was done in the month of July in a tropical 
climate, where putrefaction takes place with 
great rigidity. Yet, although many vultures 
were seen, from time to time, sailing in aU di- 
rections over the spot where the putrid carcass 
was Ijdng, coTered only with twigs of cane, none 
ever discovered it ; . but in the mean while» 
several* dogs had found their way to it, and had 
devoured lai^ quantities of the flesh. In anotiber 
set of experiments it was found that young vul- 
tures, enclosed in a cage, never exhibited aay 
tokens of their perceiving food, when it could 
not be seen by them, however near to them it 
was brought.* 

It has been doubted wh^er fishes^ and other 
aquatic animals, possess the sense of smell ; in 
some of the whale tribe, indeed^ neither the ov- 
gah of smell nor the olfactory nerves are found;t 
Some physiologists have gone the length of de- 

* Edinburgh New Journal of Science, ii. 172. The accutacy 
of these results, which had been contested by Mr. Waleitoa, is 
fully established by the recent observations and experiment of 
Mr. Bachman, which are detailed InLoudon^s Magazine of Kat, 
Hist, vii, 167." 

t Home ; Lectures on Comparatire Anatomy, i. 17. 

SM£LU 409 

Dying the capabiHty of water to serve as the ve- 
hicle of odorous effluvia. But as water is known 
to contain a lai^e quantity of air, which acts upon 
the. organs of respiration, it is easy to conceive 
tiiat it may also convey to the nostrils th6 peco- 
liar agents which are calculated to excite pdrcep^ 
tioDS of smelL Fishes are, in fact, observed to be 
attracted from great distances by the effluvia of 
snbstances thrown into the water ; and they are 
well known to have a strong predilection for all 
highly odoriferous substances. Baits used by 
ai^^rs are rendered more attractive by being 
impregnated with volatile oUs, or other sub- 
stances having a powerful scent, such as assa- 
fcetida, camphor, and musk. Mr. T. Bell^ has 
discovered in the Crocodile and Alligator, a gland, 
which secretes an unctuous matter, of a strong 
musky odour, situated beneath the lower jaw, 
on each side. The external orifice of this gland 
is a small slit, a little within the lower edge of 
the jaw ; and the sac, or cavity containing the 
odoriferous substance, is surrounded by two 
delicate bands of muscular fibres, apparently 
provided for the purpose of first bringing the 
gland into a proper position, and then, by com- 
pressing it, discharging its contents. Mr. Bell 
conceives that the use of this secretion is to act 
as a bait for attracting fish towards the sides of 
the mouth, where they can be readily seized in 

• Phil. Irani, for 1827, p. 132, ' 

410 TUB ȣN90aiilL FUNCTIONS. 

the mode uanial to the idl^atoiv which is that of 
tmapping sideways at the objeete he auns at de- 

The oi^ns ci smell in FiaheB aire sitimled ifi 
caTiliea, placed one oa each side, in front of the 
head: they are merely blind sacs, haying no 
commanication with the mouth or throat, and 
indeed no other dutlet bat the external openings^ 
which aie generally two to each sac. The prm- 
dpal ^nttance is funmihed with a Talve, fi^rmed 
b^ a moveMe mfmhnme, appearing like a paiv 
titibn dividing each nostril into two cavities, and 
serving the purpose of preventing the introdne* 
tkm ot any foreign body. The organ itsdf is 
situated behind this valve, and consists ei&er of 
a membrane, curiously plaited into numearoas 
semidrcular folds, or of tufted or arboradent 
lilalneats. Fig. 38& shows this cavity (s), with itB 

• \ 

plaited membrane in the Perch ; and F^. 389, in 
the Skate ; the laminae in the former being radi* 
ated, and in the latter, foliated, or parallel to 
each other. On the surface of these oi^ans. 

SM£LL. 411 

whaterer be their diape, the oUactwy nerves 
(m), arising from the anterior lobes (o) of the 
brain, are distriboted ; and die great size of these 
nerves would lead na to infer considerable arate- 
ness in the sense which tb^ supply. . Whan 
the fish is swimming, ibeir situation in fr<mt of 
the snout exposes them lathe forcible impulse of 
the water, which strikes against' them. Accord- 
isig to Geofiioy Sit Hikdre, the water waters the 
ca^vityby the upper orifice,! and esciBtpe^ by the 
lower. Scarpa alleges tha;t fishes exercise this 
sense by ccHnpressing the water against this 
membrane* On the other hand, it is contocided 
by Dumdril, that the peneptabns communicated 
by this organ, beiu^ the resdk tof the action of a 
liquid instead of a gas, should be classed under 
the head of taste rather than of smellv This 
se^nsy however, to be a mere verbal Gritieiud, in 
making which it appears to hare been forgotten 
that the impretoions of odwous eflSuvia, even 
in animals breathing atmospheric air, always 
act upon the nerve through the intermedium of 
the fluid which lubricates the membrane of the 

That the nasal cayities of fishes are rudimental 
forms of those of the mammalia, although they 
do not, as in the latter class, open into the. respi- 
ratory organs, is shown by the curious transform- 
ation of the one into the other during the de- 
velopement of the tadpole, both, of the frog and 


of liie salamander. During the. first periods of 
their existence, these animals are perfiectly 
aquatic, breathing water by means of gills, and 
having all their organs formed on the model of 
the fish. Their nasal cavities are not employed 
for respiration at this early period, nor even for 
some time afi^r they have begun to take in air, 
which they do by the mouth, swallowijig it in 
small portions at a time, and afterwards throwing 
it out in bubbles by the same channel. But when 
they quit the water, and become land animals 
with pulmonary respiration, the nostrils are the 
channels through which the air is received and 
expelled ; and it is here also that the sense of 
smell continues to be exercised. 

We know very little respecting the seat of the 
sense of smell in any of the invertebrated 
animals, though it is very evident that insects, 
in particular, enjoy this faculty in a very high 
degree* Analogy would suggest the spiracles sis 
the most probable seat of this sense, being the 
entrances to the respiratory passages. This 
office has, however, been assigned by many to 
the antennae; while other entomologists have 
supposed that the palpi are the real organs of 
smell^* Experiments on this subject are at* 
tended with great difficulty, and th^ results 
must generally be vague and inoonclusivie» 

* On the subject of this sense in insects. See Ktrby and 
Spence's Introduction to Entomology, vol. iv. p. 249. 

SMELL. 413 

Those which Mr. P. Huber made on bees seem, 
however, to establish, with tolerable certainty, 
that the spiracles are insensible to strong odours, 
sach as that of oil of turpentine, which is ex- 
ceeding offensive, to all insects. It was only 
when a fine camel-hair pencil containing thia 
pungent fluid was presented near the cavity of 
the mouth, above the insection of the proboscis, 
that any visible effect was produced upon the 
insect, which then gave decisive indications of 
strong aversion* Mr. Kirby has discovered in 
the anterior part of the nose of the Necraphorus 
vespittoj or burying-beetle, which is an insect 
remarkable for the acuteness of its smdl, a pair 
of circular pulpy cusjiions, covered with a mem«* 
brane, beautifully marked with fine transverse 
furrows. These he considers as the organs of 
6mell ; and he has found similar structures in 
several other insects.* 

No distinct organs of smell have been disco*- 
vered in any of the MoUusca ; but as there is 
evidence that some of the animals belonging to 
that class possess this sense, it has been cour 
jectured that it resides either in the whole 
mucous surface of the mantle^ or in the respi- 
ratory organs. Swammerdam observed, long 
ago, that snails are evidently affected by odours.; 
and cuttle-fish are said to show a decided 
aversion to strongly scented plants^ 

• Ibid. vol. iii, 481 ; and iv, 254. 


Chapter V. 


^ 1 . Acoustic Principles. 

The knowledge acquired by animals of the pre* 
sence and movements of distant objects is dep- 
rived almost wholly from the senses of hearing 
and of sight ; and the apparatus, necessary for 
the exercise of these senses, being more ela^ 
borate and refined than any of the organs we 
have yet examined, exhibit still more irre<- 
fragable evidence of those profound designs, and 
that infinite intelligence, which have guided the 
construction of every part of the animal frame. 

Sound results from certain tremulous or vi- 
bratory motions of the particles of an elastic 
medium, such as air or water, excited by any 
sadden impulse or concussion given to those 
particles by the movements of the sounding 
body. These sonorous vibrations are trans- 
mitted with great velocity through these fluids, 
till they strike upon the external ear ; and, then, 
after being concentrated in the internal passogWr 
of the organ, they are made to act on the fila* 


m^to of a particular iierVe called the acoustici 
or a$tdilofy netted of which the stractore is 
adapted to receive these peculiar iinpressions, 
and to communicate them to the brain» where 
they produce changes, which are immediately 
followed by the senisuition of sound. Sound 
cannot traverse a void space, as light does ; but 
always requires a ponderable material vehicle 
for its transmission; and, aocordingly, a bell 
suspended in the vacuum of an air-pump, gives, 
when struck, no audible sound, although its 
parts are visibly thrown into the usual vibratory 
motions* In proportion as air is admitted into 
the receiver, the sound becomes more and more 
distinct ; and if, on the other hand, the air be 
condensedf the sound is louder th$in when the 
bdU is surrounded by air of the ordinary den* 


The impulses given by the sounding body to 
the contiguous particles of the elastic medium^ 
are propi^ted in every direction, from partide 
to particle, each in its turn striking against the 
next, and communicating to it the whole of 
its own motion, which is destroyed by the re* 
action of the particle against which it strikes^ 
Hence, after moving a certain definite distance, 
a distance, indeed, which is incalculably small, 

* Thete facts were first ascertained by Dr. Hauksbee. See 
PbilosDpliicd IVansactioas for 1705, toI. xxit, p. 1902; 
19M. ' 


each particle letunis bscktoilB frcmer akaalioD, 
and 18 again ready to receive a second in^nba: 
Each particle, being elastic within a certain 
range*, saSen a momentary compression, and 
immediately afterwards resomes its fomner 
shape : the next particle is, in the mean time, 
impelled, and undei^oes the same sncc cooio B of 
dianges ; and so on throogfaoot the whole series 
of particles. Thns the sonorous nndnlaftions 
have an analogy with waves, which spread in 
circles on the surfiM^e of water, around any body, 
which by its motion ruflfes that snr&oe; only 
that instead of merdy extending in a haraontal 
plane, as waves do, the sonorous undnlationa 
q>read out in all directions, fMming, not cirdes 
in one plane, but ^[iherical shells ; and, whatever 
be the intensity of the sounds, the velocity vritfa 
which the undulations advance is unifonn, as 
long as they continue in a mediuni of nnilbna 
densi^. This velocity in air is, on an average, 
about 1 100 feet in a second, or twelve and a half 
miles in a minute: it is greater in dense, and 
smaller in rarefied air; being, in the same 
medium, exactly proporticmal to the elasticity of 
that medium. 

* The particles of water are as elastic, witluQ a liiaited dis* 
tance, as those of the most solid body, although, in consequence 
of their imperfect cohesion, or rather their perfect mobility in all 
directions, this property cannot be so easily recognised in masses 
of fluids, as it can in solids. 


Water is the medium of sound to aquatic 
animals^ as the air is to terrestrial animals. 
Sounds are, indeed, conveyed more quickly, and 
to greater distances, in water than in air, on ac* 
count of the greater elasticity of the constituent 
particles of water, within the minute distance 
required for their action in propagating sound*' 
Stones^ struck together under water, are heard 
at great distances by a person whose head is 
under water*- Franklin found by experiment 
that sound, after travelling above a mile'tfardugb 
water/ loses but little of iis intensity. According 
to Cbladnr, the velocity of sound in water is 
about 4900 feet in a second, or between four and 
five times as great as^ it is in air* 
^ SUid foodies, especially such as are hard and 
riastic, and of uniform substance, are also ex-*' 
cellent conductors of sound. Of this we may 
easily convince ourselves by applying the ear 
to the end of a log of wood^ or a l<»ig iron rod, 
in which situation we shall hear very distinctly 
the smallest scratch made with a pin at th^ 
other end ; a sound, which,, had it passed 
lliTodgh the air only, would not have been heard 
at all. In like manner, a poker suspended' by 
two strings, the ends of which are applied to the 
two ears, communicates to the organ, when struck, 
vibrations which would never have been heard 
under ordinary circumstances* It is said that 
the hunters in North America, when deaurous of 



hearing the sounds of distant fixitsteps, whFch 
would be quite inaudible in any other way, apply 
their ears close to the eardi, and then readily 
distmguish them. Ice is known to convey 
sounds^ even better than water: fdr if canium 
be fired firom a distant fort^ where a fieozen riTcr 
intervenes, each flash of li^t is followed by two 
distinct reports, the first being conveyed by the 
ice, and the second by the air. In like mann^, 
if the upper part of the wall of a high. buiMii^ 
be struck with a hammer, a person standing close 
to it on the ground, will hear two sounds after 
each blow, the first descending through the waU, 
and the second through the air. 

As sounds are weakened by difiusion over a 
larger sphere of particles, so they are capaUe of 
having their intensity increased by amcentra^ 
tion . into a smaller space ; an effect which may 
be produced by their being reflected fix>m the 
solid walls of cavities, shaped so as to bring the 
undulations to unite into a focus ; it is on this 
' principle that the ear-trumpet, fiyr assisting per- 
sons dull of hearing, is . constructed : and the 
same effect sometimes takes place in echoes, 
which occasionally reflect a sound of greater 
loudness than the original sound which was 
/ directed towards them. 

If the impulses given to the nerves of the ear 
be repeated at equal intervals of time, provided 
these intervals be very small, the impressions 


become so blended together as not to be dis- 
tinguishable from one another, and the sensation 
of a uniform continued sound, or i/Atisical fiot€j 
is excited in the mind. If the intervals between 
the vibrations be long, the note is grave ; if short, 
that is, if the number of vibrations in a given 
time be great, the note is, in the same proportion, 
acute. The former is called a low, the latter a 
hiffk note ; designations which in all probability 
w«e originally derived from the visible motions 
of the throat of a person who is singing these 
di£ferent notes; for, independently of this cir- 
cumstance, the terms of high and low are quite 
arbitrary ; and it is well known that they were 
applied by the ancients in a sense exactly the 
reverse of that in which we now use them. 
' The different degrees of tension given to the 
cord or wire of a stringed musical instniment, 
as well as its different lengths, determine the 
frequency of its vibrations ; a greater tension, or 
a shorter length, rendering them more frequent, 
and consequently producing a higher note ; and 
on the contrary, the note is rendered more grave * 
by either lessening the tension, or lengthening 
the cord or wire. In a wind instrument, the 
tone depends altogether upim the length of the 
tube producing the sound. 

There are, therefore, two qualities in sound 
recognisable by the ear, namely, loudness, or 
intensity J and quality, or tofie; the former de- 


pending on the fwce of the yibrationa ; the 
latter, on their frequency. These acoufltic pririt 
copies are to be borne in mind in studying the 
compartttive physiology of hearidg ; and »v^t 
the functions of the different parts of the ivgan 
of this sense are, as yet, but imperfectly /Unddrr 
stood, I shall, in treating of this subject, 4eyiiii|i9 
from the plan I have hitherto followed, and po«r 
laise an account of the structure of the ear in its 
most perfectly developed state, which it appears 
to be in Man. 

^ 2. Physiology of HeaHng in Man^ 

That part of the organ of hearing, which, above 
all others^ is essential to the performance of this 
function,, is the acoustic nerve, .of which the 
fibres are expanded, and spread over the surface 
of a fine membrane, placed in a situation 
adapted to receive the fidJl impressiion of the 
sonorous undulations ' which are conveyed to 
them* This membrane, then,, with its nervous 
filaments, may be regarded as- the immediate 
organ of the sense; all the other parts btiog 
merely accessory apparatus, designed to colldet 
and to condense the vibrations of the surround- 
ing medium, and to direct their concentrated 
action on the auditory membrane. 

. I liave endeavoured. Id Fig. 390, to exhibit, 
in one view, the principal parts of this cotnpU- 
cated organ, as they exist in man, in their rela- 
tive situations, and of their natural size : thereby 
(^rding a scale by which the real dimennons 
ctftbdse portions, which I shall afterwards have 
occasion to explain by magnified representations, 
may be properiy apfH^ciated.* 

• The Coaeha, or external ear (c), is formed of 
on clastic plate of cartilage, covered by inte- 
-^nent, and presenting various elevations and 
^degressions, which form a series of paraboKc 
^curves, apparently for the purpose of collectiBg 
the sonorous undulations of the air, and of di- 

* in thu and all the following Bgunt, the paiU of the right 
ear ue ihown, and, umilar paiU are ulnya indicated by the 
nine letten. 


recting them into a funnel-shaped canal (m), 
termed the meatus auditarius^ which leads to the 
internal ear. This canal is composed partly of 
cartilage and partly of bone ; and the integu- 
ment lining it is furnished with numerous small 
glands, which supply a thick oily fluid, of an 
acrid quality, apparently designed to prevent the 
intrusion of insects : the passage is also guarded 
by hairs, which appear intended for a similar 

The meatus is closed at the bottom by a 
membrane (d), which is stretched across it like 
the skin of a drum, and has been termed, from 
this resemblance, the membrane of the tympanum^ 
or the ear-drum* It performs, indeed, an office 
corresponding to its name ; for the sonorous un- 
dulations of the air, which have been collected, 
and directed inwards by the grooves of the 
concha, strike upon the ear-drum, and throw it 
into a similar state of vibration. The ear-drum 
is composed of an external membrane, derived 
from the cuticle which lines the meatus ; an in- 
ternal layer, which is continuous with that ' of 
the cavity beyond it ; and a middle layer, wioch 
consists of radiating muscular fibres, proceeding 
from the circumference towards the centre, where 
they are inserted into the extremity of a minute 

* The inner surface of the ear-drum is shown in this iisrure, 
the cavity of the tympanum, which is behind it, being laid 


bony process (h)» presently to be described.* 
This muscular structure appears designed to 
yajry the degree of tension of the ear-drum, and 
thus adapt . the rate of its vibrations to those 
communicated to . it by the air. There is also a 
slender muscle, situated internally^ which by 
acting on this delicate process of bone, as on a 
lever, puts the whole membrane on the stretchy 
and enables its radiating fibres to effect the 
nicer adjustments required for tuning, as it may 
be called, this part of the organ, f 

Immediately behind the membrane of the ear*» 
drum, there is a hollow space (t)^ called the 
cavity of the tympanum^ of an irregular shape» 
scooped out of the most solid part of the t^m-^ 
poral bone, which is here of great density and 
hardness. This cavity is always filled with air ; 
but it would obviously defeat the purpose of thd 
organ . if the air were confined in this space ; 
because unless it were allowed occasionally to 
expand or contract, it. could not long remain iii 
equilibrium with the pressure exerted by the 
atmosphere on the external surfaqe of the ear- 
drum; a pressure which, as is well known, is 
subject to great variations, indicated by the rise 
and £sill of the barometer. These variations 


* In many quadrupeds their insertion into this process is at 
some distance from the centre of the membrane. These mas- 
cular fibres are delineated in Fig. 45, vol. i, p. 136. 

t Home, Lectures, &c., iii, 268. 


Vould expose the membrane of the ear-drum to 
great inequalities of pressure at its outer and 
inner surfaces, and endanger its being fwoed^ 
ticcording to the state of the weadier, eitiiier out- 
wards or inwards^ which would cooq>letely inters 
fjsre with the d^cacy of its vibrations. Natame 
has guarded against these evils by establisluDg 
a passage of communication between the tym- 
pdnuffl and flie external air, by means of a tdbe 
(e), termed the Eustachian tube^ which begins by 
a small orifice from the inner side ^ the cavity 
of the tympanum, and opens by a wide mouth at 
the back of the nostrils.* This tube perfoDna 
the ^me office in the ear, s» the hole which it 
is found necessaiy to make in the side of adnm, 
for the purpose of opening a communication 
witli the external air; a communication which 
is as necessary for the functions of the ear, as it 
is for the proper sounding of the drum. We 
find, accordingly, that a degree of deafness is 
induced whenever the Eustachian tube m ob- 
structed, which may happen either from the 
swdling of the m^nbrane lining it, during a 
cold, or from the accumulation of secKtioii in 
the passage. It is also occasionally useful as a 
channel through which sounds may gain admit- 
tance to the internal ^r ; and it is perhaps for 

• This opeoing is seen at e, in Fig. 382, p. 400, representiug 
a vertical and longitudinal seetion of the fight nostril. 


this reason that we instinctiTely open the mouth 
when we are intent on hearing a very faint or 
£stant sound. 

On the side of the cavity of the tympanum, 
which is opposite to the opening of the Eu- 
stachian tube, is situated the beginning of 
another passage, leading into numerous cells, 
contained in the mastoid process of the temporal 
bone, and therefore termed the mastoid ceils: 
these cells are likewise filled with air. - The 
innermost side of the san>e cavity, that is the 
side opposite to the ear-drum, and' which is 
shown in Fig. 3.9), is occupied by a rounded 
eminence (p), of a triangular shape, termed the 
promotOonf; on each side of which there is an 


bpenfngin the bone, closed, however, by the 
ntembrane Kning the whote internal surface of 
the cavity. The opening (o), which is situated 
at' "the upper edge crfthe promontory, is called 
the fenestra ovalis, or oval window ; and that 
near the under edge (r), is the fenestra rotunda, 
or round window. 

Connected with the membrane of the ear- 


drum, at one end, and with the fenestra ovidis 
at the other, there extends a chain of very 
minute moveable bones, seen at (b), in Fig. 390 ; 
but more distinctly in Fig. 302, which is drawn 
on a somewhat larger scale, and in which b» 
before (d) is the ear-dnim ; (p) the promontory, 
(o), the fenestra ovalis; and (r) the fenestra 
rotunda. These bones, which may be called 
the tympanic ossictila, are four in number, and 
are represented, enlarged to twice the natural 
size, in Fig. 393. The names they have receiyed 
are more descriptive of their shape than of their 
office. The first is the fnaJleus^ or hammer (m) ; 
and its long handle (h) is affixed to the centre of 
the ear-drum : the second is the incusy or anvil 
(i) ; the third, which is the smallest in the body, 
being about the size of a inillet seed, is the orbi- 
cular bone (p)* ; and the last is the stapes^ or 
stirrup (s), the base of which is applied to the 
membrane of the fenestra ovalis. Thes^ bones 
are regularly articulated together, with all the 
ordinary apparatus of joints, and are moved by 
small muscles provided for that purpose. Their 
office is apparently to transmit the vibrations of 
the ear-drum to the men^brane of the fenestra 
ovalis, and probably, at the same time, to in- 
crease their force. 

* Blumenbach, and other anatomistB, consider this as not 
being a separate bone, but only a process of the incus ; a view 
of the subject which is supported by the observations of Mr. 
Shrapnell, detailed in the Medical Gazette, xii, 172. 


The more internal parts of the ear compose 
what is designated, from the intricacy of its wind- 
ing passages, the labyrinth. It is seen at (s v k) 

in Fig. 390, in ccmnex- 
ion with the tympanum; 
but in !Pig. 394, it is repre- 
sented, on a very large 
scale, detached from every 
other part, and separated 
from the solid bone in 
which it lies embedded. 
It consists of a middle por- 
tion, termed the vestibule 
(v), from which, on its 
upper and posterior side, proceed the three 
tubes (x, Y, z), called, from their shape, the 
semicircular canals ; while to the lower anterior 
side of the vestibule there is attached a spiral 
canal, resembling in appearance the shell of a 
snail, and on that account denominated the 
Cochlea (k). All these bony cavities are lined 
with a very delicate membrane, or periosteum^ 
and are filled with a transparent watery, or thin 
gelatinous fluid, which is termed by Breschet, 
the petnlpnph.'* 

Within the cavity of the osseous labyrinth now 
described, are contained membranes having 
nearly the shape of the vestibule and semicir- 

* Annales des Sciences Natureltes, xxix, 97. It has also 
been called the Aqua labyrinthi, and the fluid of CotunniuSf 
from the name* of the Anatomist who first distinctly described it. 


cular canals, but not esteoding into the cochlea. 
These membranes, which compose what hsB 
been tenned, for the sake of distinction, the 
membranoia labyrinth, form one coDtiouous, bat 
closed sac. containing a fluid*, perfectly Bimilar 

in appearance to the perilymph, which sur- 
rounds it on the outer side, and intervenes be- 
tween it and the sides of the osseous labyrinth, 
preventing any contact with those sides. In 

' De BlaiDville has termed this fluid " la vitrine auditive," 
from its supposed analogy with the vitreous humour of the eye. 


Fig. 395, which is on a still larger scale than 
the preceding figure, the osseous labyrinth is 
laid open, so as to show the parts it encloses, 
and more especially the membranous labyrinth, 
floating in the perilymph (p). The form of 
this latter part is still more distinctly seen, in 
Fig. 396, where it is represented in a position 
exactly corresponding to the former figure, but 
wholly detached from the bony labyrinth, and 
connected only with the nervous filaments which 
are proceeding to be distributed to its different 

A simple inspection of these figures, in both 
of which the corresponding parts are marked by 
the same letters, will show at once the form and 
the connexions of the three semicircular canals, 
(x, Y, z), each of which present, at their origin 
from the vestibule, a considerable dilatation, 
termed an ampulla (a, a, a), while, at their other 
extremities, where they terminate in the vesti- 
bule, there is no enlargement of their diameter : 
and it will also be seen that two of these canals 
(x and y) unite into one before their termination. 
The same description applies in all respects 
both to the osseous and to the membranous 
canals contained within them ; the space (p) 
which intervenes between the two, being filled 
with the perilymph. But the form of the 
membranous vestibule demands more particular 
notice, as it is not so exact an imitation of that 


of the osseous cavity ; being composed of two 
distinct sacs, opening into each other: one of 
these (u) is termed the utricle'* ; and the other 
(s), the sacculus. Each sac contains in its in- 
terior a small mass of white calcareous matter, 
(o, o), resembling powdered chalk, which seems 
to be suspended in the fluid contained in the 
sacs by the intermedium of a number of nerrous 
filaments, proceeding from the acoustic nerves (g 
and n), as seen in Fig. 396. From the universal ^ 
presence of these cretaceous substances in the 
labyrinth of all the mammalia, and from their 
much greater size and hardness in aquatic 
animals, there can be little doubt that they per- 
form some office of great importance in the phy* 
siology of hearing.! Their size and appearance 
in the Dog is shown in Fig. 397 ; and in the 
Hare, in Fig. 398. 

The Cochlea, again, is an exceedingly curious 
structure, being formed of the spiral convolu- 
tions of a double tube, or rather of one tube, 
separated into two compartments by a partition 
(l), called the lamina spiralis^ which extends its 
whole length, except at the very apex of the 

* Scarpa and Weber term it the sinus or alveus utriculostts ; 
It 18 called by others the sacculus vestibuli. Breschet ^ives it 
the name of le sinus median. See the Memoir already quoted, 
p. 98. 

t These cretaceous bodies are termed by Breschet otolithes^ 
and otoconies^ according as they are of a hard or soft consistence. 
Ibid. p. 99. 


cone, where it suddenly terminiates in a curved 
point, or hook (h), leaving an aperture by which 
the two portions x>f the tube communicate to- 
gether. In Fig. 395, a bristle (b, b) ja passed 
through this aperture* The central pillar, round 
which these tubes take two and a half circular 
turns, is termed the modiolus. Its apex is seeu 
at (m). One of these passages is distinguished 
by the name of the vestibular tube* 9 in conse- 
quence of its arising from the cavity of the ves* 
tibule ; and the other by that of the tympanic 
tube'\f because it begins from the inner side of 
the membrane which closes the fenestra rotunda, 
and forms the only separation between the 
interior of that tube, and the cavity of the tym* 
panum. The trunk of the auditory nerve occu* 
pies a hollow space immediately behind the 
ventricle, and its branches pass through minute 
b<4es in the bony plate which forms the wall of 
that cavity, being finally expanded on the dif- 
ferent parts of the membranous labyrinth.]: 

* Scala vestibuli, t Scala tympani, 

I la Fig. 396, the anterior trunk of the auditory nerve is seen 
(at g) distributing branches to the ampulloe (a, a), the utricle 
(u), and the calcareous body it contains; while the posterior 
trunk (x) divides into a branch, which supplies the sacculus (s) 
and its calcareous body (o), and a second branch (k) which is 
distributed over the cochlea, (d) is the nerve called the portio 
duray which merely accompanies the auditory nerve, but has no 
relation to the sense of hearing. In Fig. 390, the auditory 
nerve (v) is seen entering at the back of the vestibule. 


Great uncertainty prerails with regaiii to the 
real functions performed by the several parts of 
this very complex apparatus. It is most pro- 
bablci however, that the sonorous vibrations of 
the air which reach the external ear, are directed 
down the meatus, and striking against the ear- 
drum which closes the passage, throw thai mem- 
brane into vibrations of the same frequency ; to 
which the action of its muscles, which appear in- 
tended to regulate its^ tension, may also contribute. 
The vibrations of the ear-drum, no doubt, excit« 
corresponding motions in the air contained in 
the cavity of the tympanum ; which, again, com- 
municates them to the membrane of the fenestra 
rotunda; while, on the other hand, the mem- 
brane closing the fenestra ovalis, receives simihur 
impressions from the stapes, conveyed through 
the chain of tympanic ossicula, which appear t9 
serve as solid conductors of the same vibrations^ 
Thus the perilymph, or fluid contained in the 
labyrinth, is affected by each externsd sound, both 
through the taiedium of the air in the tympanum, 
and by means of the ossicula : the undulations 
thus excited produce impressions on the extre- 
mities of the nervous filaments, which are spread 
over the membranous labyrinth ; and these im- 
pressions being conveyed to the brain, are imme^ 
diately followed by the sensation of sound. 

With regard to the purposes which are an- 
swered by the winding passages of the semi- 


circular canals, and cochlea, hardly any plaus- 
ible conjecture has been offered ; yet no doubt 
^an be entertained that the uses of all these parts 
are of considerable importance, both as to deli- 
cacy and correctness of hearing. There is an 
obvious correspondence between the positions of 
the three semicircular canals, (two of which are 
vertical, and one horizontal, and of which the 
planes are reciprocally perpendicular to one ano- 
ther,) and the three dimensions by which theg^o- 
jmetrical relations of space are estLqaated ; and it 
^Slight hence be conjectured that the obje<ct of 
this arrangement is to allow of the transmission 
;of vibrations of every kind, in whatever directicH^ 
they may arrive. . It is not an improbable sup- 
{)Osition that the return into the vestibule, c^ 
4indulations which have passed through these 
canals, • has the effect of at once puttiag a 
stop to all farther motion of the fluid, and pre^ 
venting the continuance of the impression .which 
has been already made on the nerves. The 
same use may be assigned to the double spiral 
.convolutions of the tubes of the cochlea : for the 
undulations of the fluid in the tympanic tube, 
received from the membrane of the fenestra 
rotunda, will meet those proceeding along the 
vestibular tube, derived from the membrane of 
the fenestra ovalis, and like two opposing waves, 
will tend to destroy one another. Thus each 
external soupd will produce but a single mo- 



mentary hnpreagion ; the prolongation of the 
undulations of the fluid of the labyrinth being 
prevented by their mutual collision and neutral* 

§ 3. Comparative Physiology of Hearing. 

The structure of the organs of hearing in the 
lower animals presents a regular gradation from 
the simple vestibule, with its membranous sac, 
supplied with nervous filaments, which may be 
regarded as the only essential part of this organ, 
through the successive additions of semicircular 
canals, fenestra ovalis, tympanic cavity, ossicida, 
ear-drum, meatus auditorius, cochlea, and con- 
cha, till we arrive at the combination . of all 
these parts in the higher orders of the M amr 
malia. The simpler forms are generally met 

* The preliminary steps in the process above described are not 
absolutely essential to hearing, for many instances have occurred 
in which the power of hearing has been perfectly retained after 
the membrane of the ear-drum, and also the ossicula had been 
destroyed by disease. A small aperture in the membrane does 
not interfere with its power of vibration ; but if the whole ear- 
drum be destroyed, and the ossicula lost, an almost total deafness 
generally ensues. After a time, however, the hearing may be in 
a great measure recovered, with an undiminished power of dis- 
tinguishing musical tones. See two papers by Sir Astley Ck)oper, 
in the Phil. Trans, for 1800, p. 151 ; and for 1801, p. 437. 

HEAlftlNO. 43S 

with in aquatic animals^ probably because the 
donorouB undulations of water are communicated 
more readily, and with greater force, than thom 
of air, and require no accessory apparatus for 
their concentration. The lobster, for instance, 
has a vestibular cavity (seen at v, in Fig. 399), 
containing a membranous sac, with a striated 
groove (g)*, and receiving the filaments of the 
auditory nerve. This vestibule is protected by 
the shell on all sides, except at one part, where 
it is closed only by a membrane (e), which may 
therefore be conmdered as corresponding to the 
fenestra ovalis. The outer-side of this mem^ 


brane in the Astaciis fiuviatilis^ or cray-fish, is 
seen at f in Fig. 401 ; while Fig. 402, shows 
an interior view of the same membrane (f), with 
the vestibule (v) laid open, and the auditory 
nerve (n) passing through the shell to be dis- 
tributed on the sacculiis. 

It appears from a variety of observations that 
Insects, both in their larva and their perfect 

* Th» groove is represented magnified in Fig. 400. 


State, possess the faculty of hearing; but no 
certain knowledge has been obtained of the 
partSi which exercise this sense. The prevailing 
opinion among entomologists is that it resides in 
some part of the antennae; organs, which are 
supposed to have a peculiar sensibility to aerial 
undulations. This hypothesis is founded princi- 
pally on the analogy of the Crustacea, whose 
antennae contain the vestibular cavity already 
described ^ but on the other hand it is opposed 
by the fact that Spiders, which hear very acutely, 
have no antennae; and it is also reported that 
insects, when deprived of their antennae, still 
retain the power of hearing.* 

None of the MoUusca appear to possess, even 
in the smallest degree, the sense of hearing, if 
we except the highly organized Cephalopoda; 
for in them we find, at the lower part of the car- 
tilaginous ring, which has been supposed to ex- 
hibit the first rudiment of a cranium, a tubercle, 
containing in its interior two membranous vesi- 
cles, contiguous to each other, and surrounded 
by a fluid. They evidently correspond to the 
vestibular sacs, and contain each a small cal- 
careous body, suspended from the vesicles by 

* Camparetti has described structures in a great number of 
insects, which he imagined were organs of hearing; but his 
c(bservations have not been confinned by subsequent inquirers, 
and their accuracy is therefore doubtful. See De Blainville 
" De t'Qrganisation des Animaux/* i, 565. 


Blender nervous filaments, like the clapper of a 
bell, and probably performing an office ana- 
logous to that'ihstrument ; for, being thrown into 
a tremulous motion by every undulation of the 
surrounding fluid, they will strike against the 
membrane, and communicate similar and still 
stronger impulses to the nerves by which they are 
suspended, thus increasing the impression made 
on those nerves. The mechanical effect pf an 
apparatus of this kind is shown by the simple 
experiment, mentioned by Camper, of enclosing 
a marble in a bladder full of water, and held in 
the hand ; when the slightest shaking of the 
bladder will be found instantly to communicate 
motion to the marble, the reaction of which on 
the bladder gives an unexpected concussion to 
the hand. 

The ear of Fishes contains, in addition to the 
vestibule, the three semicircular canals, which 
are in general greatly developed.* An enlarged 
view of the membranous labyrinth of the Lophius 
piscatorius is given in Fig. 403, showing the form 
and complication of its parts, which are repre- 
sented of twice the natural size, x, y, z, are 
the semicircular canals, with their respective 
ampullae (a, a, a), m is the Sinns medianuSy or 
principal vestibular sac, with its anterior ex- 

* la the lamprey, these canals exist only in a rudimental state, 
appearing as folds of the membrane of the vestibule; and there 
are also no cretaceous bodies in the vestibular sac. 


pMwion, tenned the Utricle (v). The SaeaUut 
(a) has, in like manaer, a posterior appoidage 
(c) tenned the Cytticuk. The hard (nlcaieous 
bodies (o, o, o) are three in number ; and the 
branches of nerves (i, i, i) by which they axe 
suspended in the fluid cimtained in the mem- 
branes, are seen pasnng into them ; vhile the 
ampullffi are supi^ed by other branche8(N,N»M). 

In all the oBseous fishes the labyrinth is not en- 
closed in the bones of the cranium, but prcyecls 
into its cavity \ but in the larger cartilaginous 
fishes, as the ray and shark tribes, it is soi^ 
rounded by solid bone, and ia not visible vrithin 
the cranium. In these latter fishes, we first 
meet with a rudiment of the meatus, in a passage 
extending from the inner side of the vestibule, 
to the upper and back part of the skull, where 
it is closed by a membrane, which is covered by 
the skin. 

Aquatic reptiles have ears conBtructed neariy 
on the same plan as those of fishes: thus the 
Triton (»■ Newt, hag a vestibule containing only 
one cretaceous body, and three semicircular 
canids, unprotected by any surrounding Ixme. 
In the Frog, howerer, we first perceive the ad- 
dition of a distinct cavity, closed by a mem- 
brane, which is on a level with the integuments, 
on each side of the head. From this cavity, 
which corresponds to that of the tympanum, 
there proceeds an Eustachian tube ; and within 
it, extending from the external membrane, 
which mi^t here be regarded as an ear-drum, 
to the membrane of the vestibule, or fenestra 
ovalis, is found a bone, shaped like a trumps 
and termed the Columella. This bone is seen 
at o in Fig. 404, attached by its base (b) to the 
fenestra ovalis of the vestibule (v), which con- 

tains the cretaceons body (o). There is also a 
small bone (i) attached in front to the columella. 
in the Chelonia, the structure of the ear is 
essentially the same as in the Frog, but the 


tympanum and columdla are of greater length. 
In the saorian reptiles the cavity of the tym- 
panum is still more capacious, and the ear-drum 
very distinctly marked, and these animals posr 
sesB great delicacy of hearing. IThe labyrinth 
of the Crocodile is enclosed in bone, and con«^ 
tains three calcareous bodies : it pres^its also an 
appendage which has been regarded as the 
earliest rudiment of a cochlea; and there are 
two folds of the skin, resembling eye-lids, at the 
extenial orifice of the organ, which appear like 
the first step towards the devdopement of an 
external ear. 

The structure of the ear in the Crocodile is 
but an approximation to that which we find pre- 
vailing in Birds, where the organ is of large size 
compared with that of the head. The rudi- 
mental cochlea, as seen at k in Fig. 405, which 
represents these organs in the Turkey, is of 
large size, and slightly curved. In the cavity 
of the tympanum (t) is seen the columella, which 
extends to the fenestra ovalis ; and beyond it, the 
semicircular canals (s), the bony cells (b) which 
communicate with the tympanum, the os quad- 
ratum (q), the zygomatic process (z), and thci 
lower jaw (j). The ear-drum is now no longer 
met with at the surface, but lies concealed at the 
bottom of a short meatus, the orifice of which is 
surrounded with feathers arranged so as to serve 
as a kind of imperfect concha, or external ear* 


In Owls these feathers are a prominent and cha- 
racteristic feature ; and in these hirds there is, 
besides,, a membranous flap, acting as a valye to 
gwffd the passage. 

The chief peculiarity observable in the in- 
ternal ears of Mammalia is the great develope- 
ment of the cochlea, the tubes of which are con- 
Yoluted, turning in a spiral, and assuming the 
figure of a turbinated shell. From an extensive 
comparison of the relative size of the cochlea in 
different tribes of quadrupeds, it has been in- 
ferred that it bears a tolerably constant propor-r 
tion to the degree of acuteness of hearing, and 
that, consequently, it contributes essentially to 
the perfection of that faculty : bats, for instance, 
which are known to possess exquisite delicacy 
of hearing, have a cochlea of extraordinary size, 
compared with the other parts* of the ear. The 
tympanic ossicula are completely developed 
only in the Mammalia.* It is also in this class 
alone that we meet with a concha, or external 
ear, distinctly marked ; and the utility of this 
part, in catching and collecting the sonorous 
undulations of the air, may be inferred from the 
. circumstance, that a large and very moveable 
concha is generally attended with great acute- 

• These tympanic ossicula are regarded by Geoffroy St. Hiiaire 
as corresponding to the opercular bones of fishes, where, accord- 
ing to his theory, they have attained their highest degree of 


iie» of hearing. This is more paiticalariy the 
case with feeble and timid qoadnipeds, as the 
haie and rabbit, which aie erer en the watch to 
catch the most distant sounds of danger, and 
whose eais are tamed backwards, or in the 
direction of their porsneis; wiiile, on the con- 
trary, theears of predaceoos animals are directed 
forwards, that is, towards the objects of their 
pursuit. This difference in direction is not 
confined to the external ear, but is observable 
also in the iMmy passage leading to the tym* 

The Cetacea, bdng strictly inhabitants of the 
water, have no external ear; and the passage 
leading to the tympanum is a narrow and wind- 
ing tube, formed of cartilage instead of bcme, 
and haying a very small external aperture. In 
the Dolphin tribe the orifice will barely admit 
the entrance of a pin ; it is also exceedingly 
small in the Ihigong; these structures being 
eridently intended for preventing die entrance 
of any quantity of water.* It is apparently 
with the same design that in the Seal the pas- 
sage makes a circular turn; and that, in the 
OmWiorhyncus paradoxuSy it winds round the 
temporal bone, and has its external orifice at a 
great distance firom the vestibule. IThe internal 

* It is probable that in these animals the principal channel 
by which sounds reach the internal oigan is the Eustachian 


parts of the organ of hearing in the Whale and 
other cetacea, are enclosed in a bone of extra- 
CHrdinary hardness, which^ instead of forming a 
continuous portion of the skull, is connected to 
it only by ligaments, and suspended in a kind of 
osseous cavity, formed by the adjacent bones. 
The cochlea is less developed than in quad- 
rupeds, for it only takes one turn and a half, 
instead of two and a half. The existence of the 
semicircular canals in the cetacea was denied 
by Camper ; but they have since been disco- 
vered by Cuvier. 

Several quadrupeds, which are in the habit of 
burrowing, or of diving, as the Sorex fodiens^ or 
water-shrew, are furnished with a valve, com- 
posed of a double membrane, capable of accu- 
rately closing the external opening of the meatus, 
and protecting it from the introduction of water, 
earth, or other extraneous bodies.* In like 
manner the eifternal ear of the Hippopotamus^ 
which feeds at the bottom of rivers, is guarded 
by an apparatus which has the effect of a valve. 

We find, indeed, the same provident care dis- 
played in this, as in every other department of 
the animal economy : every part, however mi- 
nute, of the organ of this important sense, being 
expressly adapted, in every species, to the par- 
ticular circumstances of their situation, and to 

* Geoffrey St. HUaire ; Memoires du Museum, i, 305. 


that d^ree of acuteness of perception, which is 
best suited to their respective wants and powers 
of gratification •* - 

Chapter VI. 


§ 1. Object of the Sense of Vision. ' 

To those who study nature with a view to the 
discovery of final causes, no subject can be more 
interesting or instructive than the physiology of 
Vision, the most refined and most admirable <rf* 
all our senses. However well we may be ac* 
quainted with the construction of any particular 
part of the animal fi^une, it is evident that we 
can never form a correct estimate of the excel* 
lenca of its mechanism, unless we have also a 
knowledge of the purposes to be answered by it, 
and of the means by which those purposes can 
be accomplished. Innumerable are the works <rf 
creation, the art and contrivance of which we 

* The Comparative Physiology of the Voice, a function of 
which the object, in animals as well as in man, is to prodace 
sounds, addressed to the ear, and expressive of their ideas, feel- 
ings, desires and passions, forms a natural sequel to that of 
Hearing; but Sir Charles Bell having announced his intention 
of introducing it in his Treatise on the Hand, I have abstained 
from entering into this extensive subject. 

VISION. 446 

are incompetent to understand, because we per- 
ceive only the ultimate effects, and remain igno- 
rant of the operations by which those effects are 
produced. In attempting to inyestigate these 
obscure functions of the animal or vegetable 
economy, we might fancy ourselves engaged in 
the perusal of a volume, written in some un- 
known language, where we have penetrated the 
meaning of a few words and sentences, sufficient 
to show us that the whole is pregnant with the 
deepest thought, and conveys a tale of surpassing 
interest and' wonder, but where we are left to 
gather the sense of connecting passages by the 
guidance of remote analogies or vague conjecture. 
Wherever we fortunately succeed in decypher- 
ing any continued portion of the discourse, we 
find it characterized by a perfection of style, and 
grandeur of conception, which at once reveal 
a master's hand, and which kindle in us the 
most ardent desire of supplying the wide chasms 
perpetually intervening in the mysterious and 
inspiring narrative. But in the subject which 
now claims our attention we have been permitted 
to trace, for a considerable extent, the continuity 
of the design, and the lengthened series of means 
employed for carrying that design into execu- 
tion ; and the view which is thus unfolded of 
the magnificent scheme of creation is calculated 
to give us the most sublime ideas of the wisdom, 


On none of the works of the Creator, which 


we are permitted to behold, have the characters 
of intention been more deeply and legibly en- 
graved than in the organ of vision, where the 
relation of every part to the effect intended to 
be produced is too evident to be mistaken, and 
the mode in which they operate is at once 
placed within the range of our comprehension. 
Of all the animal structures, this is, peifaaps, the 
one which most admits of being brought into 
close comparison with the works of human art ; 
for the eye is, in truth, a refined optical instru- 
ment, the perfection of which can never be fully 
appreciated until we have instituted such a com- 
parison ; and the most profound scientific inves- 
tigations of the anatomy and physiol(^ o£ the 
eye concur in showing that the whole of its 
structure is most accurately and skilfully adapted 
to the physical laws of light, and that all its parts 
are finished with that mathematical exactness 
which the precision of the effect requires, and 
which no human effort can ever hope to ap- 
proach, — far less to attain. 

Tq the prosecution of this inquiry we are 
further invited by the consciousness of the incal- 
culable advantages we derive from the sense of 
sight, the choicest and most enchanting of our 
corporeal endowments. The value of this sense 
must, indeed, appear inestimable, when we con- 
sider of how large a portion of our sensitive 
and intellectual * existence it is the intermediate 

VISION. 447 

source. Not only has it given us extensive com- 
mand over the objects which surround us, and 
enabled us to traverse and explore the most dis- 
tant regions of the globe, but it has introduced 
us to the knowledge of the bodies which compose 
the solar system, and of the countless hosts of 
stars which are scattered through the firmament, 
thus expanding our views to the remotest con- 
fines of creation. As the perceptions supplied 
by this sense are at once the quickest, the most 
extensive, and the most varied, so they become 
the fittest vehicles for the introduction of other 
ideas. Visual impressions are those which, in 
infancy, furnish the principal means of deve* 
loping the powers of the understanding : it is to 
this class of perceptions that the philosopher 
resorts for the most apt and perspicuous illustra- 
tions of his reasonings ; and it is also from the 
same inexhaustible fountain that the poet draws 
his most pleasing and graceful, as well as his 
sublimest imagery. 

The sense of Vision is intended to convey to 
its possessor a knowledge of the presence, situ- 
ation, and colour of external and distant objects, 
by means of the light which those objects are 
continually sending ofi^, either spontaneously, or 
by reflection from other bodies. It would ap- 
pear that there is only one part of the nervous 
system so peculiarly organized as to be capable 


of being affected by luminous rays, and c<Hiyey- 
ing to the mind the sensation of light ; and this 
part is the Retina^ so named from the thin and 
delicate membranous net -work, on which the 
pulpy extremities of the optic nerves^ establish- 
ing an immediate communication between that 
part and the brain, are expanded. 

If the eye were so constructed as to allow the 
rays of light, which reach it from surrounding 
objects, simply to impinge on the retina as they 
are received, the only perception which they 
could excite in the mind, would be a general 
sensation of light, proportionate to the total 
quantity which is sent' to the organ from the 
whole of the opposite hemisphere. This, how- 
ever, does not properly constitute Vision ; for in 
order that the presence of a particular object in 
its real direction and position with respect to us, 
may be recognised, it is necessary that the light, 
which comes from it, and that light alone, pro- 
duce its imj)re5sion exclusively on some parti- 
cular part of the retina ; it being evident that if 
the light, coming from any other object, were 
allowed to act, together with the former, on the 
same part, the two actions would interfere with 
one another, and only a confused impression 
would result. The objects in a room, for ex- 
ample, are all throwing light on a sheet of paper 
laid on the floor ; but this light, being spread 
equally over every part of the surface of the 

VISION. 449 

p^er, furnishes no means of distinguishing the 
sources from which each portion of the light has 
proceeded ; or, in other words, of recognising the 
respectiye figures, situations, and colours of the 
objects themselyes. We shall now proceed to 
consider the modifications to be introduced into 
the structure of the organ, in order to attain 
these objects. 

§ 2. Modes of accomplishing the Objects of Vision, 

Let us suppose that it were proposed to us as 
a problem to invent an apparatus, by which, 
availing ourselves of the known properties of 
light, we might procure the concentration of 
all the rays, proceeding from the respective 
points of the object to be viewed, on separate 
points of the retina, and obtain likewise the ex- 
clusion of all other rays ; and also to contrive 
that the points of the retina, so illuminated, shall 
have the same relative situations among one 
another, which the corresponding points of the 
surrounding objects have in nature. In other 
words, let us suppose ourselves called upon to 
devise a method of forming on the retina a 
foithfiil ddineation, in miniature, of the external 
As it is a fundamental law in optics that th^ 




rays of light, while they are transmitted throv^h 
the same medium/ proceed in straight lines, the 
simplest mode of accomplishing the proposed end 
would be to admit into the' eye, and convey to 
each particular point of the retina, only a single 
ray proceeding directly from that part of the ob- 
ject which is to be depicted on it, and to exclude 
all other rays. For carrying this design into 
effect we have the choice of two methods, both 
of which we find resorted to by nature under 
different circumstances. 

. The first method consists in providing for 
each of these single rays a separate tube, with 
darkened sides, allowing the ray which traveiBes 
it, and no other, to fall on its respective point of 
the retina, which is to be applied at the (^^posite 
end of the tube. The most convenient form to 
be given to the surface of the retina, which is to 

be spread out to receive the 
rays from all these tubes, 
appears to be that of a con- 
vex hemisphere; and the 
most eligible distribution of 
the tubes is the placing them 
so as to constitute dtverging 
radii, perpendicular, in 
every part, to the surface of 
the retina. This arrange- 
ment will be understood by 
reference to Fig. 406, which represents a section 

VISION. 451 

of the whole organ: (t,t), being the tubes dis- 
posed in radii every where perpendicular to the 
conTex hemispherical surface of the retina (a). 
Thus will an image be formed, composed of the 
direct rays from each respective point of the 
objects, to which the tubes are directed ; and 
these points of the image will have, among them- 
BelveB, the same relative situation as the external 
objects, from which they originally proceeded, 
and which they will accordingly faithfully re- 

The second method, which is neariy the in- 
Terse of the first, ccmsists in admitting the rays 
throagh a small aperture into a cavity, on the 
c^poeite and internal side of which the retina is 
expanded, forming a concave, instead of a convex 


hemispherical surface. The mode in which this 
arrangement is calculated to answer the intended 
purpose will be easily understood by conceiv- 
ing a chamber (as represented in Fig. 407), into 


which DO light is allowed to enter, except what 
is admitted through a small hole in a shutter, 
so as to fall on the opposite side of the room. 
It is evident that each ray will, in that case, 
illuminate a different part of the wall, and that 
the whole external scene will be there faith- 
fully represented; for the several illuminated 
points, which constitute these images, preserve 
among themselves the same relative situation 
which the objects they represent do in nature, 
although with reference to the actual . objects 
they have an inverted position. This inversion 
of the image is a ncfcessary consequence of the 
crossing of all the rays at the same point; 
namely, the small aperture in the shutter, 
through which they are admitted. 

One inconvenience attending the limiting of 
the illumination of eac^ point of the wall to that 
of a single ray, in the mode last pointed out, is 
that the image produced must necessarily be 
very faint. If, with a view of remedying this 
defect, the aperture were enlarged, the image 
would, indeed, become brighter, but would 
at the same time, be rendered more indistinct, 
from the intermixture and mutual interfer^ice 
of adjacent rays ; for all the lines would be 
spread out, the outlines shaded off, and the whole 
picture confused. 

The only mode by which distinctness of image 
can be obtained with increased illumination, is 

VISION 453: 

to collect into one point a great number of irtys 
proceeding from the corresponding point of the 
object to be represented. Such a collection of 
rays proceeding from any point, is termed, in 
the language of optics, a pencil of rays ; and the 
point into which they are collected is called a 
focus. For the purpose of collecting a pencil of 
rays into a focus, it is evident that all of them, 
except the one which proceeds in a straight line 
from the object to that focus, must be deflected^ 
or btent from their rectilineal course. This effect 
may be produced by refraction^ which takes 
place according to another optical law ; a law 
of which the following is the exposition. 

It is only when the medium which the rays 
are traversing is of tmifonn density that their 
course is constantly rectilineal. If the density 
change, or if the rays pass obliquely from one 
medium into another of a different density, they 
are refracted ; each ray being deflected towards 
a line situated in the medium of greatest density^ 
and drawn from the point where the ray meets 
the new medium, perpendicular to the refracting 
surface. Thus the ray r, Fig. 408, striking ob- 
liquely on the surface of a denser mediuAi, at the 
point s, instead of pursuing its original course 
along the line s o, is refracted, or turned in the 
direction s t, which is a line situated between s o, 
and s p ; this latter line being drawn perpen- 
dicularly to the surface of the medium, at the 


pMDt s, and within that medium. When the 
my arrives at r, and meets the poetezior ssr- 

Ace t^ the dense medium, pasang thence into 
one that is less dense, it is again refracted 
according to the same law; that is, it inclines 
towards the perpendicniar line t i, drawn firran 
T, within the denser mediom, and describes th6 
new course t v instead of T t. lie amomit 
of the deflection corresponds to the d^ree of 
obliquity of the ray to Ihe sur&ce wfaidh re- 
fracts it ; and is mathematically expieased, by 
tbfC law, that the nnes <^ the two angles formed 
with the perpendicular by the incident and the 
refracted rays retain, amidst all the variations of 
those angles, the same constant propMiion to me 
another. We may hence derive a umple rule 
for placing the plane of the refracting surfooe so 
as to produce the paiticolar refraction we wish 
to obtain. When a ray is to be d^ected from 
its original course to a particular side, we have 
only to torn the surface of the medium in such 
a manner as that the perpendicular line to that 


surface, contained within the daiser medium, 
shall lie still farther oa the same side. Thus, in 
Fig. 408, if we wish to turn the ray r s, from 
B o to s T, we must place the dense medium so 
that the perpendicular s p, which is within it, 
shall he still farther from s o, than s t is ; that 
is, shall lie on the other side of st. The same 
rule applies to Uie contrary refraction of the ray 
8 T from TV to T u, when it passes out of a dense, 
into a rare medium ; for the perpendicular t i 
must still be placed on the same side of t v as 
T V is situated. 

XjOt us now apply these principles to the case 
befwe us ; that is, to the determination of the 
iorm to be giren to a dense medium, in order to 
collect a pencil of rays, proceeding from a distiuit 

object, accurately to a focus. We shall suppose 
the object in question to be very remote, so that 
the rays composing the pencil may be consi- 
dered as being parallel to each other; for at 
great distances their actual deviation firom 
strict parallelism is wholly insensible; and let 


A, B, c, D, E, (Fig. 409), represent these myB, 
There must evidently be one of these rays (c), 
and only one, which, by continuing its rectilineal 
course, would arrive at the point (r) intended to 
be the focus of the rays. This ray, then, may 
be sufiered to pass on, without being subjected 
to any refraction ; the surface of the medium 
idiould, therefore, be presented to the ray (at i) 
p^pendicularly to its course, so that it may pass 
through at right angles to that surface. Those 
rays (b and d) which are situated very near to 
this direct, or central ray (c), will require but a 
small d^ree of refraction in order to reach the 
focus (r) : this small refraction will be effected 
by a slight degree of obliquity in the medium at 
the points (h and k) where those rays meet it 
In proportion as the rays (such as those at a and 
e) are more distant from the central ray, a 
greater amount of refraction, and consequentiy a 
greater obliquity of the surfaces (g and l) will 
be required^ in order to bring them to the same 

The convergence of these rays, after they have 
passed this first surface, may be further increased 
by interpottug new surfaces of other media at 
the proper angles. If the new medium be still 
denser than the last, the inclination of its sur- 
face must be similar to that already described ; 
if rarer, they must be in an opposite direction. 
This last case is illustrated in the figure^ wher6 

VISION. 457 

H, N, o, p, Q, represent the inclinations of the 
surfaces of a rarer medium, calculated to in- 
crease the convei^ence of the rays, that is to 
bring. them to a nearer focus (f). The result 
of the continued change of direction in the 
reiVacting surface, is a regular currilineal sur- 
face, which, in the present case, approaches very 
dearly to that of a sphere. Hence by giving 
these lefractive media spherical surfaces, ve 
adapt them, with tolerable exactness, to produce 
the convergence of parallel rays to a focus, and 
by making the denser medium convex on both 
sides (as shown in Fig. 410), both surfaces will 

conspire in producing the desired effects. Such 
an instrument is termed a double convex lens; 
and it has the property of collecting into a focus 
rays proceeding from distant points.* 

Having obtained this instrument, we may now 

* The refraction by apherical surfaces does not, strictly speak- 
ing, unite a pencil of parallel or divergent rays into a mathe- 
matical point, or focus; for in reality the rays which are near 
the central line are made to conrei^ to a point a little more 


TCDture to enlarge the aperture through whicn 
the light was admitted into our dark chamber, 
and fit into the aperture a double ctMivex lens. 
We haye thus coDstructed the well known op- 
tical instrument called the Canura Obacura, in 
which the images of external objects are formed 
upon a white surface of paper, or a semi-trans- 
parent plate of glass; and these images must 
evidently be in an inverted position with re- 
spect to the actual objects which they re- 

Such is precisely the constructicm of the eye, 
which is, to all intents, a camera obscura : for 
in both these instruments, the objects, the prin- 

distant than that to which the remoter raja convei^ : an eflect 
which 1 have endeavouTed to illustrate by the diagram 1^.41 1 ; 
where, iu order to render it obvious to the eye, the disparity 
it much exaggerated. But on ordinary occaiionB, where great 

nicety is not required, this difference in the degree of convergence 
between tlie central rays and those near the circumference of the 
lens, giving rise to what is termed llie Aberration of SphericUy, 
is too email to attract notice. 


ciptes of construction, and the mode of operation 
are exactly the same; and the only difference 
is, that the former is an infinitely more perfect 
instrument than the latter can ever be rendered 
1^ the utmost eff<Nrts of human art. 

With a view of sinplifyins tlM^ nibjoct, I h«v« aMumed, in 
Die account giren in the text, that the reys which arrive at the 
eye are parallel, which in mathematical UrictDess they never, 
are. Hie focus of the my* refracted by a eoavez lens is more 

remote in proportion as the nys are more divergent, or, in other 
words, proceed from nearer objects. This is illustrated by 
Figures 413, 413, and 414; to which I shall again have occa- 
sion to refer in the sequel. 


§ 3. Stmctnre of the Eye. 

One of the many points of superiority which the 
eye possesses over the ordinary camera obscnra 
is derived from its spherical shape, adapting the 
retina to receive every portion of the images 
produced by refraction, which are themselves 
curved : whereas had they been received on a 
plane surface, as they usually are in the camera 
obscura, a considerable portion of the image 
would have been indistinct. This spherical form 
is preserved by means of the firm membranes 
which protect the eye, and which are termed 
its Coats; and the transparent media which 
they enclose, and which effect the convergence 
of the rays, are termed the Humours of the Eye. 
There are in this organ three principal coats, and 
three humours, composing altogether what is 
called the Glohe of the Eye. Fig. 416, which 
gives an enlarged view of a horizontal section 
of the right eye, exhibits distinctly all these 

The outermost coat (s), which is termed the 
Sclerotica, is exceedingly firm and dense, and 
gives to the globe of the eye the mechanical sup- 

VISION. 461 

port it requires for the performance of its deli- 
cate functionB. It is perforated behind by the 
optic nerve (o), which passes onwards to be ex- 
panded into the retina (r). The sclerotica does 
not extend farther than about four-fillhs of the 
globe of the eye ; its place in front being sup- 
plied by a transparent convex membrane (r.)^ 

called the Cornea, which is more prominent than 
the rest of the eye-ball. A line passing through 
the centre of the cornea and the centre of th6 
globe of the eye is called the axis of the eye. 
The Sclerotica is lined internally by the Choroid 
coat (x), which is chiefly made up of a tissue of 


blood yeaeels, for supplying noorisfament to the 
eye. It has on its inner surface a layer of a dark 
edoured yiscid secretion, known by the name of 
the Pigmentum nigrum, or Uack pigment. Its 
use is to absorb all the light which may hiq[>peii 
to be irregulariy scattered through the eye, in 
consequence of reflection from difierent quarters ; 
and it serves, therefore, the same purpose as the 
black paint with which the inside of optical in- 
struments, such as telescopes, microscopes, and 
earners obscurse, is darkened. Within the pig- 
mentum nigrum, and almost in immediate con- 
tact with it^, the RetinA (r) is expanded, form- 
ing an exceedingly thin and delicate layer of 
nervous matter, supported by a fine membrane. 
More than three-fourths of the globe of the 
eye are filled with the vitreous kumour (v), which 
has the appearance of a pellucid and elastic 
jelly, contained in an exceedingly delicate tex- 
ture of cellular substance. The Ctystalline 
kumaurj (l) which has the shape of a douUe 
convex lens, is formed of a denser material than 
any of the other humours, and occupies the fore* 
part of the globe of the eye, immediately in finont 
of the vitreous humour, which is there hollowed 
to receive it. The space which intervenes be* 

* Between the pigmentum and the retina there is found a very 
fine membrane, discovered by Dr. Jacobeon: its use has not 
been ascertained. 


VISION. 403 

tinreai the lens and the cornea is filled with a 
watery secretion (a), called the Aqueous humour. 
This space is divided into an anterior and a pos- 
terior chamber by a flat circular partition (i), 
termed the Iris. 

The iris has a central perforation (p), called 
the Pupili and it is fixed to the edge of the cho* 
roid coat, by a white elastic ring (q\ called the 
Ciliary Ligament. The posterior surface of the 
iris is called the Uvea, and is lined with a dark 
brown pigment. The structure of the iris is very 
peculiar, being composed of two layers of con- 
tractile fibres ; the one, forming concentric cir- 
cles ; the other, disposed like radii between the 
outer and inner margin.^ When the former act, 
the pupil is contracted ; when the latter act, the 
breadth of the iris is diminished, and the pupil 
is, of course, dilated. By varying the size of the 
pupil the quantity of light admitted into the 
interior of the eye is regulated, and accommo- 
dated to the sensibility of the retina. When the 
intensity of the light would be injurious to that 
highly delicate organ, the pupil is instantly con- 
tracted, so as to exclude the greater portion; 
and, on the contrary, when the light is too 
feeble, it^is dilated, in order to admit as large a 
quantity as possible. The iris also serves to in- 

• See Fig. 47. vol. i, p. 136. 


tercept such rays as would have &lIeD on parts 
of the crystalline lens less fitted to produce their 
T^^lar refraction, the object of which will be 
better understood when we have examined the 
functions of this latter part. But, before engag- 
ing in this inquiry, it will be proper to complete 
this sketch of the Anatomy of the Eye by 
describii^ the principal parts of the apparatus 
belonging to that organ, which are exterior to 
the eye-ball, and may be considered as its ap- 

The pnrposes answered by the parts exterior 
to the eye-ball are chiefiy those of motion, of 
lubrication, and of protection. 

As it is the central part of the retina which is 
endowed with the greatest share of sensibility, 
it is necessary that the images of the objects to 
be viewed should be made to fall on this part; 
and consequently that the eye should be capa- 
ble of having its axis instantly directed to those 
objects, wherever they may be situated. Hence 
muscles are provided within the orbits, fw effe<^- 
ing the motions of the 
eye-ball. A view of these 
muscles, with their attach- 
ments to the ball of the 
eye, but separated from 
the other parts, is given 
in Fig. 416. Four of these 
proceed in a straight course from the bottom of 

VIMON. 465 

the orbit, arising from tb6 mai^n of the apertare 
through which the optic nferve passes, and being 
inserted by a broad tendinous expansion into the 
fore^part of the sclerotic coat. Three of these 
are marked a, b, and c in the figure : and the 
edge of the fourth is seen behind and above b. 
These straight muscles^ as they are called, sur^ 
round the optic nerve and the eye-ball, forming 
four l<»igitudiiial bands ; one (a) being situated 
above for the purpose of turning the eye up^ 
wards ; a second (c), situated below, for turning 
H downwards ; and the two others, on either side, 
for performing its lateral motions to the right or 
left. The cavity of the orbits being con&aderably 
larger than the eye-ball, the intervening space, 
especially at the back part, is filled up by 
fat, which serves as a soft cushion for its pro^ 
tection, and for enabling it to roll freely in all 

Besides these straight muscles, there are also 
two others (s aiid i) termed the oblique muscles^ 
which give the eye-ball a certain degree of rota* 
tion on its axis; When these act in conjunction, 
they draw the eye forwards, and serve as anta* 
gonists to the combined power of the straight 
muscles. The upper oblique muscle (s) is re<- 
markable for the artificial manner in which its 
tendon passes through a cartilaginous pulley (p) 
in the margin of the orbit, and then turns back 
again to be inserted into the eye^ball, so that the 



effect produced by the action of the muscle is a 

motion in a direction exactly the reverse of that 

in which its fibres contract- This mechanirai, 

simple as it is, affords one of the most palpable 

instances that can be adduced of express c<m* 

trivance ; for in no other situation could the muscle 

have been so conveniently lodged as within the 

eye-ball ; and in no other way could its tendon 

Imve been made to pull in a direction contrary to 

that of the muscle, than by the interposition of a 

pulley, turning the tendon completdy round. 

The fore-part of the globe of the eye, which is 
of a white colour, is connected with the sur* 
rounding int^uments by a membrane, termed 
the Comnmcii^'^ This membrane, on arriving 
at the tese df the eye-lids, is folded forwards so 
as to liit^ their inner surfaces, and to be con« 
tinuous with the skin which covers their outer 
^jj^. The surfaces of the conjunctiva and €i 
llie cornea are kept constantly moist by the 
tears, which are as constantly secreted by the 
Jjocrynud glands. Each gland, (as shown at 
L, Fig 417,) is situated above the eye, in a hol- 

* An abundant supply of nenres hat been bestowed on this 
membrane for the purpose of confening upon it that exquisite 
degree of sensibility which was necessary to give immediate warn- 
ing of the slightest danger to so important an organ as the eye 
from the intrusion of foreign bodies. That this is the intention 
is apparent from the fact that the internal parts of the eye 
possess but little sensibility compared with the external surface. 

VISION. 467 

low of the orbit, and the ducts (d) proceeding 
from it open upon the inner side of the upper 
eye-lid (e). This fluid, the uses of which are 
obviously to wash away dust, or other irritating 
substances which may happen to get introduced, 
is distributed over the outer sur^ce of the eye 
by means of the eye-lids. Each lid is sup- 

ported by an elastic plate of cartilage, shaped 
like a crescent, and covered by integuments. 
An orbicular muscle, the fibres of which run in 
a circular direction, immediately underneath the 
skin, all round the eye*, is provided for closing 
tbem. The upper eye-lid is raised by a separate 
muscle, contained within the orbit, immediately 
above the upper stra^ht muscle of the eye-ball. 

* See Fig. 46, vol. i, p. 136. 


The eye-lashes tire curved in opposrie dUreciidlid, 
80 as not to int^ere with each other when tiie 
eye-lids are closed. Their utility in gutt^dHn^ 
the eye against the entrance of various suh- 
stances, such as hairs, dust, or persjiiratioil, ^md 
also in shading the eye from too stlmig ^n^ 
pressiens of li^t^ is suflSciently apparMtt* The 
eye-lids, in closing, meet first at the outer 
comer of the eye ; and their junction proceeds 
along the line of their edges, towards the innar 
angles, till the contact is complete: by this 
means the tears are carried onwards in that 
direction and accumulated at the inner coma 
of the eye, an efiect which is promoted by the 
bevelling of the margins of the eye-lids, which, 
when they meet, form a channel for the fluid to 
pass in that manner. When they arrive at the 
inner comer of the eye, the tears are conveyed 
away by two slender ducts, the orifices of which, 
called the puncta lacrymalia (p, p), are seen at 
the inner comer of each eye-lid, and are sepa- 
rated by a round projecting body (c), connected 
with a fold of the conjunctiva, and termed the 
laayfnal oai^ncle. Tiie two ducts soon unite to 
form one passage, which opens into a *sac (s), 
utuated ajt the upper part of the sides of the 
nose, and terminating below (at n) in the cavity 
pf the nostrils, into which the tears are ulti- 
mately conducted. When the secretion of the 
tears is too abundant to be carried off by 

viaiON. 469 

channel, they overflow upon th^ cheeks; but 
when the quantity is not e^ceiksive, the ten;* 
dency to flow ov» the ejre*li:d ia checls^ed by an 
oily secretion proceeding from a row of minute 
glands, situated at the edge of the eye-lids, and 
termed the MeibomiaH ghmds^. 

The eye^brows are a farther pix>tection to the 
eyes, the direction of the hairs being such as 
to turn away from them any drops of rain or 
of perspiratioA which may ch9^c§ to fall fro^ 

Excepting ia^ fro^t, where the ^y es are covered 
and protected by the eye-lids, these important 
QKgans are on all ^ides efiectuaJly guarded from 
ixy[iiry by be^ng contained in a hollpw bony 
socket, termed the orbit, and. composed of seven 
^rtions of bone. These seven elements may be 
ree<^nised in the skuUs of all the mammalia, 
and perhaps also in those of all other verter 
brated anin^als^ afibrding a remarkable illustra- 
tion of the unity of the plans of nature in the 
construction of the anioial fabric. 

§ 4. Physiology^ of perfect Visi 

The rays of light, proceeding flrom a distant 
oLyect, strike upon the convex surfecje of the 
cornea, which being ^ greater denaUy than the 


air, refracts them, and makes them conveige to^ 
wards a distant focus. This effect, however, is 
in part counteracted on their emergence from 
the concave posterior surface of the cornea, 
when the rays enter into the aqueous humour. 
On the whole, however, they are refracted, and 
made to converge to a degree equal to that 
which they would have undergone if they had 
at once impinged against the convex surface of 
the aqueous humour, supposing the cornea not to 
have been interposed. 

A considerable portion of the light which has 
thus entered the aqueous humour is arrested in 
its course by the iris ; so that it is only thos^ 
rays which are admitted through the pupil that 
are subservient to vision. These next arrive at 
the crystalline lens, where they undergo two re- 
fractions, the one at the anterior, the other at 
the posterior surface of that body. Both these 
surfaces being convex outwardly, and the lens 
being a denser substance than either the aque- 
ous or the vitreous humours, the effect of both 
these refractions is to increase the convergence 
of the rays, and to bring them to unite in a focus 
on the retina at the bottom of the eye. The 
most considerable of these refractions is the 
first ; because the difference of density between 
the air and the cornea, or rather the aqueous 
humour, is greater than that of any of the hu* 
mours of the eye compared with one smother. 

VISION. 47 1 

The accurate convergence of all the rays of 
light, which enter through the pupil, to their 
respective foci on the retina, is necessary for the 
perfection of the images there formed ; hut for 
the complete attainment of this end various nice 
adjustments are still requisite. 

In the first place, the Aberration of Sphericity* ^ 
which is a consequence of the geometrical law 
of refraction, introduces a degree of confusion in 
the image ; which is scarcely perceptible, indeed, 
on a small scale, but which becomes sensible in 
instruments of much power ; being one of the 
greatest difficulties which the optician has to 
overcome in the construction of the telescope and 
the microscope. Nature, in framing the human 
eye, has solved this difficulty by the simplest, 
yet most efiectual means, and in a manner quite 
inimitable by human art. She has in the first 
place given to the surfaces of the crystalline 
lens, instead of the spherical form, curvatures 
more or less hyperbolical or elliptical ; and has, 
in the next place, constructed the lens of an 
infinite number of concentric layers, which in- 
crease in their density, as they succeed one ano- 
ther from the surface to the centre. The refract- 
ing power, being proportional to the density, is 
thus greatest at the centre, and diminishes as we 
recede from that centre. This admirable ad- 

* See Fig. 411, and the note referring tie it, p. 457. 

472 THE SENSOftliia' fUNCTIONS. 

justment exactly coirects the deficiency of re* 
ftaction, wiiich always take^ place in die central 
portions of a lens composed of a material of 
tmiform density, as compared with th^ refiactiQii 
of the partSi more remote from the centre.* 

The second adjuetment for perfect vision has 
reference to the variatioDif in the distance of 
|he focuik which t^ke place according as the 
mys arrive at the eye froiQ objects at diffbre^ 
distances,' 9xid which may be called the Aberrq^ 
fio9^ €^ PoraHlaof. When tJ^ie distance of the 
object i& Tery great, tb€^ r9,ys proQeeding from 
each ppint; arrive a^; the^ ^ye with £M» little 
divef gencf ^ ^at ei^ch pei^cU n^^y b^ condidere4 
as composed of ray» -which aore parallel to eai:i| 
other; the actual deviation from parallelism 
being quite insensible. But if thb aame object 
be brought nearer \o the eye, the diveigeqce of 
th^ rayi» be^n^ mpr^ perceptible ; un4 t^ 
^fect of the san^ degree of refraction is to 
eeilect them into ^ focus mor^ reipipote than 
before.t F<^ every distance pf the QJ^ct tki»e 

* Sir Dai^ Bvewster has ascertained that the vai^atioiis of 
density producing the doubly refracting stnictuse, in the crp^ 
talline lens of fishes, are related, not to the centre of the lens, 
but to the diameter which forms the axis of vision : an arrange* 
mcvl peculiarly adapted for correctisg the spherical aberratio^B^ 
Pbiloa. Trans, for 1816, p. 317. 

t This is illustrated by Pig. 412, 413, and 414; the first of 
which shows the rapid convergence of rays proceeding from a 
very distant object, and which may be considered as parallel. 

YIWON. 473 

ia ft ootre^iKNidug fop^ 4i8toAee ; and wl^ tb^ 
eye ^ ia a st^te acb^ted £Df ^Usti^Qt visuwi q^ 
9Qe 4i»teBee, it will have confused im^ge^ of 
P^i^te ^ aootkep di0taa€4^; Veeau«^ tbje exact 
foci of ih^ r^ys will 1^ aitUAted eitlvef he^^ of 
))0hii]4 tbe retina.. It ifi^ ^ident ttjat, if tlau^ 
Keti^a be npt placed e«Mctly at tJ^iB point irb^re 
th« £9C|is ip situate^* i^ wi^l either interiQ^pt ih^ 
pencil of yay« b^ore, tbiey are united, in^ «fe poinU 
o/t ree^ye them after they ha^ cffo^s^d one 
avotber in passing through th^ foou0 :, ia ^ther 
of which ca9es, each peucU will ikww upon the 
retina a awall di^cl^ of lighti bn^ter at the 
middle a^d fo^i^ter at the edfg^ which li^ill mkn 
i^ts^ lirith ^he adja^^ept penetts, apc| e«^at^ con^^ 
fafliOQ 19 the imag^* 

It is fouqdy however, tb^at the ^e ha^ a powev 
of aoeommodating itself to tt^ distipct viaioft of 
OJ^ijeets at a great variety of distaae^s, aocordiag 
W t^a atteatiaa of th^ qtii^td is div^ed to the 
pwticvilar object ta be^fnrad- The «aode^ ia 
iKhiph Hm change in the state of the ^^ ia 
«4^e^ted hfW beei^ the smbject qf much ^ontfOr 
veray . Th^, in^i;ease of the ifefjfacting pav«« af 
the eye aecessiary to adapt it to the visioQ of no4r 
oJb^ect^ IS evidently the. result of a pi4a?^ar 
a^foFt, oif which we ve distinctly ^oaaci^ua w^h^a 

The second shows that divergent rays ui^ite at a more distant 
focus ; and the thirds that the focas is more distant the greater 
th» diverg^ooe. 


we accurately attend to the accompanying sen^ 
sations. The researches of Dr. Young have ren* 
dered it probable that some change takes place 
in the figure of the lens whereby its convexity, 
and perhaps also its distance from the retina, are 
increased. He has shown by a yery decisive 
experiment, that any change which may take 
place in the convexity of the cornea has but 
little share in the production of the effect ; for 
the eye retains its power of adaptation when im* 
mersed in water, in which the form of the cornea 
can in no respect influence the refraction. 

But the rays of light are of different kindi ; 
some exciting the sensation of red, others of 
yellow, and others again of blue; and these 
different species of light are refracted^ under 
simUar circumstances, in different degrees. 
Hence the more refrangible rays, that is the 
violet and the blue, are brought to a nearer 
focus, than those which are less refrangible, that 
is the orangte and the red rays : and this want 
of coincidence in the points of convergence o£ 
these different rays, (all of which enter into the 
composition of white light), necessarily impairs 
the distinctness of all the images produced by 
refraction, shading off their outlines with various 
colours, even when the object itself is colourless. 
This defect, which is incident to the power of a 
simple lens, and which is termed the Chromatic 
Aberratum^ is remedied almost perfectly in the 

VISION. 475 

^y^y l>y the nice adjustment of the powers of the 
different refracting media, which the rays of 
light have to traverse before they arrive at the 
retina, producing what is called an achramatit 
combination* ; and it is found that the eye^ 
though not an absolutely achromatic instrument, 
as was asserted by Eulert, is yet sufficiently so 
for all the ordinary practical purposes of life« 

The object, then, of the whole apparatus ap- 
pended to the optic nerve, is to form inverted 
images of external objects on the retina, which, 
as we have seen, is the expanded extremity of 
that nerve* That this effect is actually pro*- 
duced, may be easily shown by direct obser* 
vation ; for if the sclerotic and choroid coats be 
carefully dissected off from the posterior part of 
the eye of an ox, or any other large quadruped, 
leaving only the retina, and the eye so prepared 
be placed in a hole in a window-shutter, in a 
darkened room, with the cornea on the outside^ 
all the illuminated objects of the external scene 
will be beautifully depicted, in an inverted posi- 
tion, on the retina. 

Few spectacles are more calculated to raise 
our admiration than this delicate picture, which 

* For the exposition of the principles on which these achro- 
matic combinations of lenses correct this source of aberration, I 
niust refer to works which treat professedly on Optics. 

t For the rectification of this error we are indebted to Dr. 


BAKure h»». witb sxkoh exquisite art» aad with the 
|kQe«t tpucbea. of ber pencil, spread ovep the 
flWPQtb c^RTass of this subtle o^rve ; a pjiclwe^ 
wbicb^ tboitgb scwrcely owHpyiDg a apace of 
hftlf Ml ioQh in diameteri conta]:DS the deline^ 


atioo of a bouodlesa scese of earth and sky, fuU 
ol all kJAdli of objact;3» wim at. res^ and otbeva 
in laotiop, yet all accurately rej^:esent«d a^ to 
tbeil^ fonB8» coloiurs and positions,, and foUoved 
im aU thi^ir changes, without the least intev* 
fe]^0ce, irragidanty, or confusion. EYery on? 
of those cowRtlesa a«d stupendous orbs oS fire^ 
whose light, after tvaven^ing immeasurable re* 
gtona of space, aft Irogth reaches, our eyot ia cc4t 
^ted on its narrow curtain iirtoa liwiinoua focua 
of ineoaeeivahle minuteness; and yet this al^ 
mmi. VBi&nitmn»l point shall be sufficient t* 
couvey to the yiind, through the medium^ e^ tbf 
opAic menre and brain,, a knowle^e of the exist- 
M/ee. a^d positioii ci^ the 1^ distant luminis^^ 
iroest whiiQh that light has emanated. How iq£tr 
mikely swfvassbg sll the linuta of our conception 
must be the intelligence, and the- power of th^t 
Beings who plasosted and execvrted aok instruiqent 
comprising, within au€b limited dinensionf^ SMeh 
vast powers as the eye, of which the perceptions 
comprehend alike the nearest and most distant 
objects, and take cognizance at once of the meet 
minute portions of matter, and of bodies of the 
largest magnitude I 

VISION. 477 

^ b. Comparative Physiology of Vi$io9t. 

In the fmtniitioH of evei^ part of the aniwal 
biBchin^ry we may generally discern the predcH 
minance of the law of gradation ; but this law 
is more especially observed ki those organs 
which exhibit, in their most perfect state, the 
greatest cbmjdieation and refin^nent of stnic- 
tHire ; for on following all their varieties in tl^ 
tiscending series, we always find them advancing 
by slow gradations of improvement, Jb^ore tbey 
tirttain their highest d^ree of exceUenoe. l^ms 
the organ of virion presents, amidst an iiifitiite 
variety of ccmstructions, succc^ssive • degrees of 
refinement, accompanied by correspondmg ex- 
tensions of power. So gradual is the progress of 
this developiement, that it is not eai^ to determine 
the point where the feoulty of vision, properly so 
called, begins to be exercised, or where the firet 
rudiment of its organ begins to appear. 

Indications of a cettain degree of sensibility to 
light aie afforded by many of the lower tribesof 
Zoop/hyies, while no visible organ appropriated 
to receive its impressions can be traced. This is 
the case with many microscopic animalcules; 
and still more remarkably with the Hydra^ and 
the Actinittj which show by their movements that 


they feel the mfluence of this agent ; for, when 
confined in a vessel, they always place them* 
selves, by preference, on the side where there is 
the strongest light.* The Veretillum cynaouniumy 
on the other hand, seeks the darkest places, and 
contracts itself the moment it is exposed to 
light.t In A perfectly calm sea, the Medusa 
which are rising towards the surface, are seen to 
change their course, and to descend again^ as 
soon as they reach those parts of the water which 
receive the full influence oi the sun's rays, and 
before any part of their bodies has come into 
contact with the atmosphere.t But, in all these 
instances a doubt may arise whether the ob- 
served actions may not be prompted by the m^e 
sensation of warmth excited by calorific rays 
which accompany those of light ; in which case 
they would be evidence only of the operation of 
a finer kind of touch. 

The first unequivocal appearance of visual 
organs is met with in the class of Annelida; 
although the researches of Ehrenberg would 
induce us to believe that they may be traced 
among animals yet lower in the scale; for 
he has noticed them in several of the more 
highly organized Infusoria, belonging to the 

* Such ii the uniform report of Trembley, Baker, Boiiiiet> 
Goeze, Hanow, Roesel, and Schceffer. 

t Rapp ; Not. Act. Acad. Nat. Cur. of Bonn, xir, 645. 
t Grant ; Edio. Journal of Science : No* 20« 

VISION. 47d 

pider Rotifera, and particularly in the Hydatina 
senta, where he has found the small black points, 
observable in other species, united into a single 
spot of larger size^ Nitsch, also, states that the 
Cercaria viridis^ possesses three organs of this 
kind. Planaria present two or three spots, 
which have been regarded as visual organs ; and 
these have been found by Baer to be composed, 
in the Pkmaria tarva^ of clusters of black grains^ 
situated underneath the white or tranqmrent in^ 
tegument.* The eyes of the Nai$ proboscidea are 
composed, according to Gruithuisen, simply of a 
small mass of black pigment, attached to the 
extremity of the optic nerve t ; and organs ap« 
parently similar to these are met with in many 
of the inferior tribes of Annelida. In all these 
cases it is a matter of considerable doubt whe* 
ther the visual organs are constructed with any 
other intention than merely to convey general 
sensations of light, without exciting distinct per^ 
qeptions of the objects themselves from which 
the light proceeds ; this latter purpose requiring, 
as we have seen, a special optical apparatus of 
some degree of complexity. An approach to 
the formation of a crystalline lens takes place in 

* Nov. Act. Acad. Nat. Cur. of Bonn, xiii, 712. See also 
the Memoir of Dug^s, entitled ** Recherches lur rOrganisation 
et les Moenrs dea Planaires/* in the Annales des Sc. Nat. xv. 

t Nov. Act. Acad. Nat. Cur. of Bonn, xi, 242. 


Ae genud Eunice of Cuvi^r, (£^cori^»Say.) wlodl, 
from the account gtiren by Profeesol* MnUer*, 
kas four eyes, situated on die hinder part of tiie 
bead) and <>o^red ivith tbe epidermis, foi]il cm- 
tainiDg ift <lieir interior a s{dberule, composed of 
an opaque white subMance, surrounded by a 
black pigment, and penetrated by an c^tic 
nerve,^ which is continu<^d to the brain. On the 
otheflp hand, Pnrfessor Weber found in die HitnjAo 
nudicinahSf or common leech, no less than tea 
minute eyes, arranged in a semicircle, in fro&t 
of die head, and projeieting a little from the ^vlh 
fetee of the integument : they present extemalQr 
a con'^ex, and perfectly transparent cornea; 
while intei^aUy, they are prolonged into cylifi^ 
drioal tubes, c3ontaining a black pigmentf ; 
skiictures, appar^itly subservient to a species 
'6f vision of a higher order than that which con* 
sists in the simple recognition of the presence of 

No organs having the most distant relation to 
the sense of vision, have ever been observed in 
any df the Acephalous, or bivalve Mollusca ; bat 
various species of Gasteropoda have organs 
which appear to exercise this sense, situated 
sometimes at the base, sometimes at the middle, 
and frequently at the extremity of the tenta- 

* Annales des Sciences Nafturalleg, xxii, 23. 
t Meckel, Archiv flir Anatoniie und Physiologic; 1824, 
p. 301. 

VISION. 481 

cula. Of the latter we have examples in the 
common slug and snail, where these tentacula, or 
horns, are four in number, and are capable of 
being protruded and again retracted, by folding 
inwards like the finger of a glove, at the pleasure 
of the animal. According to M\iller,* the eye of 
the Helix potnatia, represented at e, (Fig. 418), 
is situated a little to one ^ide of the rounded 
extremity, or papilla (p), of the tentaculum, and 

is attached to an oval bulb of a black colour. 
It receives only a slender branch (o) from a 
large nerve (n n) which is distributed to the 
papilla of the tentaculum, and appears to be ap- 
propriated exclusively to the sense of touch. 
The bulb, with the eye attached to it, is repre- 
sented, in this figure, as half retracted within the 
tubular sheath of the tentaculum (s s) ; but it 
can exercise its proper function only when fully 
exposed, by the complete unfolding and protru- 
sion of the tentaculum. This eye contains, 
within its choroid coat, a semi-fluid and per- 
fectly transparent substance, filling the whole of 

* Annales des Sciences NaUirelles ; xxii. 12. 


the globe; and Muller also discovered at the 
anterior part, another transparent body^ hffriiig 
the shape of a lens.* A structure very similar 
to this was found to exist in the eye of the Murex 
tritaniSf with the addition of a distinct iris, per^ 
forated so as to form a pupil ; a part wlubh had 
also been observed, together with a crystalline 
lens of very large size, in the Valuta cymbium^ 
by De Blainville-t Thus the visual organs of 
these Gasteropoda appear to possess every re- 
quisite for distinct vision, properly so called. 
Experiments are said to have been recently made, 
both by Leuchs, and by Steifensand,]: in which 
a snail was repeatedly observed'to avoid a small 
object presented near the tentaculum; thus 
affording evidence of its possessing this sense. 

The accurate investigation of the anatomy of 
the eyes of insects presents considerable diffi- 
culty, both from the minuteness of their parts 
and from the complication of their structure ; so 
that notwithstanding the light which has recently 
been thrown on this interesting subject by the 
patient and laborious researches of entomologists, 
great obscurity still prevails with regard to the 

* MuUer thus confinnsthe accuracy of Swamroerdam's account 
of the anatomy of the eye of the snail, which had been contested 
by Sir £. Home (Phil. Trans. 1824, p. 4) and other writers. 

t Principes d* Anatomic Comparee, i, 445. 

\ Quoted by Muller; ibid, p. 16. These results also corro- 
borate the testimony of Swammerdam, who states that he had 
obtained proofs that the snail could see by means of these 

VISION. 483 

mode in which these diminutive beings exercise 
the sense of vision. Four descriptions of visual 
oigans are met with in the class of Articulated 
animals ; the first are the simple eyes, or stem-^ 
matUj as they are termed, which appear as lucid 
spots, resembling those we have noticed in the 
higher orders of Annelida ; the second, are the 
conglomerate eyes, which consist of clusters or 
^g^^eg^tions of simple eyes ; the third, are the 
compound eyes, which are formed of a vast 
assemblage of small tubes, each having its re* 
spective apparatus of humours and of retina, and 
terminating externally in a separate cornea, 
slightly elevated above the general surface of 
the organ : the fourth kind of eyes, which have 
not yet been distinguished by any particular 
appellation, are constituted by a number of 
separate lenses, and subjacent retinae, but the 
whole covered by a single cornea common to 
them all. 

Few insects are wholly destitute of visual 
organs, either in their larva or perfect states.^ 
The larvae of those insects which undei^o a com- 
plete metamorphosis have only stemmata; but 
those which are subjected only to a partial 
change of form, as the Orthoptera, the Hemip- 

* This is the case, however, with the genus Claviger^ ampng 
the Coleoptera ; Braula (Nitzch) among Diptera, and also some 
of the species of Pupipara, Nycteribia, and MelopkaguSy which 
are all parasitic insects : there are also five species of ants, whose 
neuters have no eyes. (Mulier, Annales des Sc. N#t. xvii. d66») 


tera, and the aquatic Neuroptera, have com^ 
pouud as well as simple eyes. Perfect insects, 
with the few exceptions aboye noticed, have 
always compound eyes, generally two in num- 
ber, placed on the sides of the head : and they 
are often accompanied by stemmata situated 
between, or behind them, on the upper part of 
the head. These stemmata, when met with, are 
generally three in number, and are either placed 
in a row, or form a triangle. Their structure 
has been minutely examined by Professer Muller, 
who found them to contain a hard and spherical 
crystalline lens, a vitreous humour, and a choroid 
coat, with its accompanying black pigment ; the 
whole being covered externally by a convex 
cornea. The stemmata of a caterpillar, which 
has eight of these eyes, are shown in Fig. 419, 

connected together by . a circular choroid mem- 
brane (x x) common to the whole ; together with 
the separate branches (o o) of the optic nerve 
(n) belonging to each. 

All the Arachnida possess eyes of this latter 
description; and from their greater size afford 

VISION. 485 

facilities for dissection, which are not met with 
among proper insects. Their number in Spiders 
is generally eight, and they are disposed with 
great symmetry on the upper side of the head. 
Fig. 420 represents, on a magnified scale, one of 
the large stemmata, on the head of the Scorpio 
tunensisj dissected so as to display its internal 
parts ; in which are seen the cornea (c), derived 
from an extension of the integument (i) ; the 
dense spherical crystalline lens (l) ; the choroid 
coat, with its pigment (x),* forming a wide open- 
ing, or pupil ; the vitreous humour (v), covered 
behind by the retina (r), which is closely ap- 
plied to it ; and the optic nerve (o), with which 
the retina is continuous. 

Examples of the conglomerate eye occur in 
the Myriapoda : in the Scolopendra^ for instance, 
they consist of about twenty contiguous circular 
pellucid lenses, arranged in five lines, with one 
larger eye behind the rest, which Kirby com- 
pares to a sentinel, or scout, placed at some little 
distance from the main body. In the Julus 
terrestrisy or common Millepede, these eyes, 
amounting to 28, form a triangle, being disposed 
in seven rows, the number in each regularly 
diminishing from the base to the apex ; an 
arrangement which is shown in Fig. 421 .f 

* Marcel de Serres states, that some of the stemmata of the 
insects which he examined contain a thin choroid, having a sil- 
very lustre, as if intended as a reflector of the light which falls 
on it. 

t Kirby and Spence's Introduction, &c,, iii. 494. 


The compound eyes of insects are formed of a 
yast nmnber of separate cylind'ers or elongated 
cones,* closely packed together on the surface 
of a central bulb, which may be considered as 
a part of the optic nerve ; while their united 
bases or outer extremities constitute the surface 
of a hemispherical convexity, which often occa* 
pies a considerable^ space on each side of the 
head. The usual shape of each of these bases is 
that of a hexagon, a form which admits of their 
uniform arrangement with the greatest economy 
of space, like the cells of a honey-c(Hnb; and 
the hexagonal divisions of the surface are very 
plainly discernible on viewing the surfitce ^ 
these eyes with a microscope, especially as there 
is a thin layer of black pigment interveniDg 
between each» like mortar between the layers of 
brick. The appeanince they present in the 
MehUmthay when highly magnified, is shown in 
Fig. 422.t The internal structure of these eyes 
will be best understood from the section of that 

* The number of these cones or cylinders which compose the 
entire organ differs much in different species. In the ant, there 
are only 50 ; in a Scarab€Bus, 3180 ; in the Bombyx marif 6236 ; 
in the house-fly (Musca domestica), 8000 ; in the MehUmtkA 
vulgariSf 8820 ; in the Phalena cobsus, 1 1 ,300 ; in the LibellulOj 
12,544; in the PapUio, 17,325; and in the Mordella^ 25,088. 

t In the Phalena, and other tribes, they are arranged in 
squares (as shown in Fig. 423), instead of hexagons, and fre- 
quently much less regularly; as must necessarily happen, in 
many parts^ from the curvature of the spherical surface. 

of the Libellula md^aia, or grey Dragon-fly, 
shown in Fig. 424, aided by the highly magni- 
fied Tiews of smaller portions given in the buc- 
ceeding figures, in all of which the same 
letters of reference are used to indicate the same 
objects.* The whole outer layer (cc) of the 

compound eye may be considered as corres- 
ponding to the cornea : each separate division of 
which has been termed a Comeule, being com- 
posed of a homy, and perfectly transparent 
material. Each comeule (c) has the form of a 
truncated pyramid, the length of which (l) is 
between two and three times the diameter of the 
base (ji). The outer surface (b) is very convex ; 
but the internal, or truncated end (d) is con- 
cave ; and the concavity of the latter being 

* These %ures, aa well as the account of the analomy of the 
eye of the IJbellula, are taken from the memoir of Dug^, in the 
Aunalea des Sciences Naturellefl, xx. 34). 


smaller than the convexity of the former, its 
optical effect is that of a meniscus, or coacavo- 
convex lens, with power of conTetging to a dis' 
tant focus the rays of light which traverse it. 

Within these comeules there is extended a layer 
of an opaque hlack pigment (x), probably con- 
nected with a choroid coat, which, from the deli- 
cacy of its texture, has hitherto escaped obser- 
vation. There exists opposite to the centre, or 
axis of each comeule, a circular perforation (p), 
which performs the functions of a pupil.* Dug^ 
states, indeed, that he has witnessed in this part 

* This pupillary aperture was discovered by MnUer, aati had 
eluded till the efibrts of former observers to detect it ; and it was 
accordingly the prevailing notioa that the black pigment lined 

VISION. 489 

moyements of contraction and dilatation, like 
those of the iris in vertebrated animals. He has 
likewise found that there is a small space (a) 
intervening between the extremity of each cor- 
neule and the iris, and filled with an aqueous 
humour. The compartments formed by the sub- 
stance of the choroid (x) are continued inwards 
towards the centre of the general hemisphere, 
the cylindrical spaces which they inclose being 
occupied each by a transparent cylinder (v), 
consisting of an outer membrane, filled with a 
yiscid substance analogous to the vitreous hu- 
mour. Their general form and situation, as 
they lie embedded in the pigment, may be seen 
from the magnified sections; each cylinder 
commencing by a rounded convex base, imme- 
diately behind its respective pupil, and slightly 
tapering to its extremities, where it is met by 
a filament (n) of the optic nerve ; and all these 
filaments, after passing for a certain distance 
through a thick mass of pigment, are united to 
the large central nervous bulb (g, Fig. 427), 
which is termed the optic ganglion^ 

the whole surface of the cornea, and interposed an insuperable 
barrier to the passage of light beyond the cornea. It was evi- 
dently^ impossible, while such an opinion was entertained, that 
any intelligible theory of vision, with eyes so constructed, could 
be formed. 

* Numberless modifications of the forms of each of these con- 
stituent parts occur in different species of insects. Very frt- 


It thus appears that each of the constitiiait 
eyes, which compose this yast aggregate, con- 
sists of a simple tube, furnished with all the ele- 
ments requisite for distinct vision, and capable 
of receiving impressions from objects situated ia 
the direction of the axis of the tube. The raj^ 
traversing adjacent comeules are prevented from 
mixing themselves with those which are proper 
to each tube by the interposition of the black 
pigment, which completely surrounds the trans- 
parent cylinders, and intercepts all lateral <Hr 
scattered light Thus has nature supplied the 
want of mobility in the eyes of insects, by the 

quently the vitreous humour (v), instead of forming an elon- 
gated cyliuder, has the shape of a short cone, terminating in a 
fin^ point, as shown in Fig. 426. Straus Durckheim appears U 
have mistaken this part for an enlarged termination of the optic 
nerve, believing it to be opaque, and to form a retina applied to 
the back of the corneule, which latter part he considered as pro- 
perly the crystalline lens. In his elaborate work on the ana- 
tomy of the Melolontha, he describes the filaments (f) of the op> 
tic nerve, in their progress inwards, as passing through a second 
membrane (k. Fig. 428), which he denominates the common 
choroid, and afterwards uniting to form an expanded layer, or 
more general retina {n\ whence proceed a small number of 
short but thick nervous columns (n), still converging towards the 
large central ganglion (g), in which they terminate. The use be 
ascribes to this second choroid is to intercept the light, which, 
in so diminutive an organ, might otherwise penetrate to the gene- 
ral retina and produce confusion, or injurious irritation. The 
colour of the pigment is not always black, but oflen h^ a bluish 
tint : in the common fly, it is of a bright scarlet hue, resemUiog 
blood. In nocturnal insects the transverse layer of pigment 
between the corneule and the vitreous humour is absent. 

VISION. 491 

vast multiplication of their number, aud by pro- 
viding» as it were, a separate eye for each sepa- 
rate point which was to be viewed ; and thus 
has she realized the hypothetical arrangement, 
which suggested itself in the outset of our in^ 
quiries, while examining all the possible modes 
of effecting this object. 

This mode of vision is probably assisted by 
the converging powers of each comeule, although 
in parts which are so minute it is hardly pos- 
sible to form an accurate estimate of these 
powers by direct experiment. In corroboration 
of this view I am fortunately enabled to cite a 
valuable observation of the late Dr. WoUaston, 
relative to the eye of the Astaciis fluviatilisy or 
cray-fish, where the length of each component 
tube is short, compared with that of the Li- 
bellula. On measuring accurately the focal 
distance of one of the corneules Dr. WoUaston 
a^ertained that it corresponds with great exact- 
ness to the length of the tube attached to it; 
so that an image of an external object is formed 
precisely at the point where the retina is placed 
to receive it.* 

Little is known of the respective functions of 
these two kinds of eyes, the simple and the com- 

* This iDterestiDg fact was communicated to me by Captain 
Kater, who, together with Mr. Children, assisted Dr. Wollaston 
in this examination. 


pound, both of which are generally possessed by 
the higher orders of winged insects. From the 
circumstance that the compound eyes are not 
developed before the insect acquires the power 
of flight, it has been inferred that they are more 
particularly adapted to the vision of distant ob- 
jects ; but it must be confessed that the expe- 
riments made on this subject have not, hitherto, 
led to any conclusive results. Dug^s found, in 
his trials, that after the stemmata had been 
covered, vision remained apparently as perfect 
as before, while, on the other hand, when in- 
sects were deprived of the use of the compound 
eyes, and saw only with the stemmata, they 
seemed to be capable of distinguishing nothing 
but the mere presence or absence of light. 
Others have reported, that if the stemmata be 
covered with an opaque varnish, the insect loses 
the power of guiding its flight, and strikes 
against walls or other obstacles : whereas if the 
compound eyes be covered while the stemmata 
remain free, the insect generally flies away, 
rising perpendicularly in the air, and continuing 
its vertical ascent as long as it can be followed 
by the observer. If all the eyes of an insect 
be covered, it will seldom make any attempt 
whatsoever to fly. 

The eyes of insects, whether simple or com- 
pound, are immoveably fixed in their situations ; 
but the compound eyes of the higher orders of 

VISION. 493 

the class Crustacea, are placed at the ends 
of moveable pedicles, so as to admit of being 
turned at pleasure towards the objects to be 
viewed-* This, however, is not the case with 
the Entomastracaj comprising the various species 
of Manoculij in which the two eyes are brought 
so close to one another as apparently to consti- 
tute a single organ, corresponding in its struc- 
ture to the fourth class of eyes already enume- 
rated; that is, the separate lenses it contains 
have a general envelope of a transparent mem« 
brane, or cornea. Muscles are provided for 
moving the eye in its socket ; so that we have 
here indications of an approach to the structure 
of the eye which prevails in the higher classes 
of animals. There is, however, a still nearer 
approximation to the latter in the eye of the 
Cephalopoda; for SepiiB differ from all the 
tribes belonging to the inferior orders of mollusca 
in having large and efficient eyes, containing a 
hemispherical vitreous humour, placed imme- 
diately before a concave retina, and receiving in 
front a large and highly convex crystalline lens, 
which is soft at its exterior, but rapidly increases 
in density, and contains a nucleus of great hard- 
ness; there is also a pigmentum nigrum, and a 

* Latreille describes a species of Crab, found on the shores of 
the Mediterranean, having its eyes supported on a long jointed 
tube, consisting of two articulations, which enables the animal 
to move them in various directions, like the arms of a telegraph. 


distinct iris, with a kidney-«haped pupil* This 
eye is remarkable for the total absence c£ a 
cornea ; the integuments of the head b^ng 
continued over the iris, and reflected ovar the 
edges of the pupil, giving a covering to the ex- 
ternal surface of the lens ; there is, of course, 
no chamber for containing an aqueous bumoon 
The globe of the eye is nearly spherical, but the 
sclerotica is double, leaving, at the posterior part, 
between its two portions, a considerable space, 
occupied by the lai^e ganglion of the optic 
nerve, with its numerous filaments, which are 
embedded in a soft glandular substance.* 

The eyes of Fishes differ from those of sepiae 
principally in the addition of a distinct cornea, 
exterior to the lens and iris, but having only a 
slight degree of convexity. This, indeed, k the 
case with all aquatic animals; for, since the 
difference of density between the cornea and 
the external medium is but small, the refractive 
power of any cornea, however convex, would be 
inconsiderable; and the chief agent for per- 
forming the requisite refraction of the mys is 
the crystalline lens. We accordingly in general 
find the cornea nearly flat, and the globe of the 
eye approaching in shape to a hemisphere; 
while the lens itself is nearly spherical, and of 

* See Cuivier, sur les MoUusques ; Memoir sur le Poulpe^ 
p. 37. In the Octopus there are folds of the skin, which appear 
to be'rudiments of eye-lids. 

VISION. 495 

great density. These oircumstances are ahown 
in the section of the eye of the Perch j Fig. 430.* 
The flatness of the cornea leaves scarcely any 
space for aqueous humour, and but little for 
the motions of the iris. 

The surface of the eye in fishes, being con* 
tinually wacdied by the water in which it is 

immersed, requires no provision 
of a secreted fluid for that pur* 
pose ; and there are consequent^ 
ly neither lacrymal apparatus* 
nor proper eye-lids ; the int^u^ 
ments supplying only a thin 
transparent membrane, which 
passes over and protects the cornea, serving the 
ofiice of a conjunctiva. The eye retains its form 
by the support it receives from the sclerotic coat, 
which is of extraordinary thickness and density. 
In the Shark and the Skate the eye is supported 
from the bottom of the orbit, by a cartilaginous 
pedicle, which enables it to turn as on a pivot, 
or lever. 

Sir David Brewster has recently made an in- 
teresting analysis of the structure of the crystal- 

* In this figure, as in the others, c is the cornea ; l, the lens ; 
y, the vitreous humour; r, the retina; o, the optic nerve; and 
s, the sclerotica. There is also found in the eyes of most fishes an 
organ, lodged in the space k, termed the Choroid gland , which 
envelopes the optic nerve^ is shaped like a horse-shoe, is of a 
deep red colour, and highly vascular; its use is quite un* 


line lens of the Cod, to which he was led by 
noticing some remarkable optical appearances 
presented by thin layers of this sabatance when 
transmitting polarised light. He found that the 
hard central portion is composed of a succession 
of concentric, and perfectly transparent, sphe- 
roidal laminae, the surfaces of which, tfaongh 
apparently smooth, have the same kind of iri- 
descence as mother-of-pearl, and arising &om 
the same cause ; namdy, the occurrence of re- 
gulariy arranged lines, or ttrite.* These lines, 
which mark the edges of the separate fibres 
composing each lamina, convei^e like meridians 
from the equator to the two poles of the sphe- 

roid, as is shown in Fig. 431. The fibres tliem- 
selves are not cylindrical, but fiat; and they 
taper at each end as they approach the points of 
convergence. The breadth of the fibres in the 
most external layer, at the equator, is about the 
6,500th of an inch. The observation of another 
optical phenomenon, of a still more delicate kind, 

• See toI, i. p. 232. 

VISION. 497 

*ed Sir David Brewster to the farther discovery 
of the curiods mode in which, (as is represented 
in Fig. 432^) the fibres are locked together at 
their edges by a series of teeth, resembling 
those of rack- work. He found the number of 
teeth in each fibre to be 12,500; and as the 
whole lens contains about 5,000,000 fibres, the 
total, number of these minute teeth amounts to 

Some fishes, which frequent the depths of the 
ocean, being found at between three and four 
liundred fathoms below the surface, to which it 
is impossible that any sensible quantity of the 
light of day can penetrate, have, like nocturnal 
quadrupeds, very large eyes-f In a few spe- 
cies, which dwell in the muddy banks of rivers, 
as the decilia^ and Munena cteca^ or blind eel, 
the eyes are quite rudimental, and often nearly 
imperceptible; and in the Gastrohranchus^ De 
Blainville states that it is impossible, even by 
the most careful dissection, to discover the least 
trace of eyes. 

Reptiles, being destined to reside in air as 

* As far as his observations have extended, this denticulated 
structure exists in the lenses of all kinds of fishes, and likewise in 
those of birds. He has also met with it in two species of Lizards ^ 
and in the Ormthorhyncus ; but he has not been able to find it 
in any of the Mammalia^ not even in the Cetacea. (Phil. Trans, 
for 1833, p. 323.) 

t See ** Observations sur les Poissons recueillis dans un Voy- 
age aux lies Bal^res et Pythiuses. Par M. Delaroche.** 



*well as in water, have eyes accommodated to 
these variable circumstances. By the protnisdon 
of the cornea, and the addition of an aqueous 
humour they approach nearer to the spherical 
form than the eyes of fishes ; and the lens has a 
smaller refractive power, because the principal 
refraction is now performed by the cornea and 
aqueous humour. Rudiments of eye«lids are 
met with in the Salamandetj but they are not of 
sufficient extent to cover the whole surface of 
the eyes. In some serpents, the int^uments 
pass over the globe of the eycf, forming a transpa- 
rent conjunctiva, or external cornea, behind which 
the eye-ball has free motion. This membrane 
is shed, along with the cuticle, every time that 
the serpent is moulting; and at these epochs, 
while the cornea is preparing to detach itself, 
air insinuates itself underneath the external 
membrane and renders it opaque: so that until 
this operation is completed and an entire sepa- 
ration effected, the serpent is rendered blind. 
Serpents have no proper eyelids ; but the cor- 
nea is covered by a transparent integument, 
which does not adhere to it.* Lizards have 

* It was the general opinion, until very lately, that serpents 
are unprovided with any lacrymal apparatus ; but a small la- 
crymal passage has been recently discovered by Cloquet, leading 
from the space in the inner corner of the eye, between the trans- 
parent integument and the cornea. This lacrymal caaal opens 
into the nasal cavity in venomous snakes, and into the mouth in 
those that are not venomous. 

VISION. 499 

usually a Angle perforated eye-lid^ ^hich, wheli 
cloeed by itd orbicular muscle, exhibits merely a 
borizontal slit There is also a small internal 
fold, forming the rudiment of a third eye-lid. 
The Chamelion has remarkably prq|ecting eyes, 
to which the light is admitted through a very 
minute perforation in the skin constituting the 
outer eye-'lid. This animal has the perwer of 
turning each eye,, independently of the oth», in 
a great variety of directions. 

The eyes of Tortoises exhibit an approach 
to those of birds : they are furnished with large 
htcrymal glands, and with a very moveable 
memlrana nictitans or third eye-lid. 

Birds present a still further developement of 
aU these parts : their eyes are of great Sf2e com^ 
pared with the head, as may be seen from the 
large portion of the skull which is occupied on 
each Side by the orbits*. The chief peculiar ities 
of the internal structure of these oi^ans are ap- 
parently designed to accommodate them to vision 
through a very rare medium^ and to' procure their 
ready adjustment to objects situated at very dif^ 
ferent distances. The form of the eye appears 
calculated to serve both these purposes ; for the 
great prominence of its anterior portion, which 
has often the shape of a short cone, or cylinder, 
prefixed to the front of a hemispherical globe, 
and which is terminated by a very convex cornea, 
affords space for a larger quantity of aqueous 


humour, and also for the removal of the l^is 
to a greater distance from the retina, whereby 
the vision of near objects is facilitated, while at 
the same time the refracting powers are suscep- 
tible of great variation. 

For the purpose of preserving the hemisphe- 
rical form of the sclerotica, this membrane in 
birds is strengthened by a circle of bony plates, 
which occupy the fore-part, and are lodged 
between the two layers of which it consists. 
These plates vary in number from fifteen 
to twenty, and they lie close together, their 
edges successively, overlapping each other. 
There is manifest design in this arrangement: 
for it is clear that a ring formed of a number of 
separate plates is better fitted to resist fracture 
than an entire bony circle of the same thick- 

There is a dark-coloured membrane, called the 
Marsupium, situated in the vitreous humour, the 
use of which is unknown, though it appears to 
be of some importance, as it is found in almost 
every bird having extensive powers of vision.* 
The comparative anatomy of the eye offers, 
indeed, a great number of special structures of 

* It is shown at m, Fig. 433, which is a magnified section of 
the eye of a Goose, c is the cornea ; i, the iris; p, the ciliary 
processes, s, the sclerotic coat, and o, the optic nerve. 

which we do not understand the design, and 
which I have therefore purposely omitted to 
notice, as being foreign to the object of this 

In most birds the membrana nictitans, or third 
eye-lid, is of considerable size, and consists of a 
semi-transparent fold of the conjunctiva, lying, 
when not used, in the inner comer of the eye, 
with its loose edge nearly, vertical : it is repre- 
sented at N, Fig. 434, covering half the surface 
of the eye : its motion, like that of a curtain, is 
horizontal, and is effected by two muscles : the 
first of which, seen at q, in Fig. 435, is called 
from its shape the quadratus, and arises from the 
upper and back part of the sclerotica : its fibres 
descending in a parallel course towards the optic 

nerve, where they terminate, by a semi-circular 
edge, in a tubular tendon. This tendon has no 


particular attachment, but is employed for the 
purpose of serving as a loop for tlie passage of 
the long tendon of the second muscle (p), which 
is called the pyramidalis^ and which arises from 
the lower and back part of the sclerotica. Its 
tendon (t), after passing through the channel 
above described, which has the effect of a pulley, 
is conducted through a circular sheath, furnished 
by the sclerotica to the under part of the eye, 
and is inserted into the lower portion of the 
loose edge of the nictitating membrane. By 
the united action of these two muscles, the 
former of which serves merely to guide the 
tendon of the latter, and increase the velocity of 
its action, the membrane is rapidly drawn over 
the front of the globe. Its return to its former 
position is effected simply by its own elasticity, 
which is sufficient to bring it back to the inner 
corner of the eye. If the membrane itself had 
been furnished with muscular fibres for effecting 
this motion, they would have interfered with its 
use by obstructing the transmission of light. 

The eyes of quadrupeds agree in their g^ieral 
structure with those of man. In almost all the 
inferior tribes they are placed laterally in the 
head, each having independent fields of visdon, 
and the two together commanding an extensive 
portion of the whole sphere. This is the case 
very generally among fishes, reptiles, and birds. 
Some exceptions, indeed, occur in particular 

VISION. 503 

tribes of the first of these classes, as in the 
UranascopuSf where the eyes are directed imme- 
diately upwards; in the Matf and the Callio- 
nymuSj where their direction is oblique ; and in 
the PleurmiecteSy where there is a remarkable 
want of symmetry between the right and left 
sides of the body, and where both eyes, as well 
as the mouth, are apparently situated on one 
side. Among birds, it is only in the tribe of Owls, 
which are nocturnal and predaceous, that we find 
both eyes placed in front of the head. In the 
lower quadrupeds, the eyes are situated laterally, 
so that the optic axes form a . very obtuse angle 
with each other. As we ascend towards the 
quadrumana we find this angle becoming 
smaller, till at length the approximation of the 
fields of view of the two eyes is such as to 
admit of their being both directed to the same 


object at the same time. In the human species 
the axes of the two orbits approach nearer to 
parallelism than in any of the other mammalia ; 
and the fields of vision of both eyes coincide 
nearly in their whole extent. This is probably 
a circumstance of considerable importance Math 
regard to our acquisition of correct perceptions 
by this sense. 

In the magnitude of the organ compared with 
that of the body, we may occasionally observe 
some relation to the character of the animal and 
the nature of its pursuits. Herbivorous animals, 


and especially those whose bulk \A greats as 
the Elephant^ the Rhinoceros^ and the Hippo- 
potamus^ have comparatively small eyes; for 
that of the elephant does not exceed two 
inches in diameter. The eye of the Whale is not 
much more than the 200th part of the length 
of the body. When the natural food of an 
animal is stationary, and requires no effort of 
pursuit, the eye is generally small, and the 
sight less keen ; while in the purely camivorous 
tribes, which are actively engaged in the chase 
of living prey, the organ of vision is large and 
occupies a considerable portion of the head ; the 
orbit is much developed, and encroaches on the 
bones of the fisu^e ; while, at the same time, the 
bony partition separating the globe of the eye 
from the temporal muscle is supplied by ligament 
alone : so that when that muscle is in strong 
action, the eye is pressed outwards, giving to 
the expression of the countenance a peculiar 

While nature has thus bestowed great acute- 
ness of sight on pursuing animals, she has, 
on the other hand, been no less careful to arm 
those which are the objects of pursuit, with 
powers of vision, enabling them to perceive 
their enemies from afar, and avoid the impend- 
ing danger. Thus, large eyes are bestowed 
on the Rodentia and the Ruminantia. Those 
tribes which pursue their prey by night, or 


VISION. 605 

in the dusk of the evening, as for example 
the Lemur and the Caty are furnished with 
large eyes. Bats, however, form an exception 
to this rule, their eyes being comparatively 
small; but a compensation has been afforded 
them in the superior acuteness of their other 
senses. In many quadrupeds a portion of the 
choroid coat is highly glistening, and reflects 
a great quantity of coloured light: the object 
of this structure, which is termed the Tapetum^ 
is not very apparent. 

Among the lesser quadrupeds which burrow 
in the ground, we find many whose eyes are 
extremely minute, so much so, indeed, as to be 
scarcely serviceable as visual organs. The eye 
of the SoreXj or shrew mouse, is very small, and 
surrounded by thick hair, which completely 
obstructs vision, and requires to be removed 
by the action of the subcutaneous muscles, 
in order to enable the animal to derive any 
advantage from its eyes. These organs in the 
Moh are still more remarkably deficient in 
their developement, not being larger than the 
head of a pin, and consequently not easily 
discovered.* It is therefore probable that this 
animal trusts chiefly to its sense of hearing, 

* Magendie asserts that the mole has no optic nerve ; but G. 
St. Hilaire and Carus recognise the existence of a very slender 
nervoas filament, arising from the brain, and dbtribated to the 
eye of that animal. 


ifi^hich is remarkably acute, for intimatioDS of 
the approach of danger, especially as, m its 
subterranean retreats, the vibrations of the solid 
earth are readily transmitted to its ears. The 
Mus typhlus^ or blind rat of Linnaeus; (the 
Zemni of Pallas,) which is an inhabitant of 
the western parts of Asia, cannot be supposed 
to possess even the small degree of visioQ of 
the mole : for no external oigan of this s^ise 
has been detected in any part of that animal. 
The whole side of the head is covered with 
a continuous int^ument of uniform thickness, 
and equally overspread with a thick velvetty 
hair. It is only after removing the skin that 
a black spot is discovered on each side, of ex- 
ceedingly small size, and apparently the mere 
imperfect rudiment of an eye, and totally in- 
capable of exercising any of the functions of 

Those mammalia whose habits are aquatic, 
having the eye frequently immersed in a dense 
medium, require a special provision for accom- 
modating the refractive power of that oi^;an to 
this variation of circumstances. Accordingly it 
is found that in the Seal^ and other amphibioiis 
tribes, the structure of the eye approaches to 
that of fishes, the lens being denser and moie 
convex than usual, the cornea thin and yidd- 
ing, and both the anterior and posterior s^- 
ments of the sclerotic thick and firm; but 

VISION. 507 

the middle circle is very thin and flexible, 
admitting of the ready separation or approxi- 
mating of the other portions, so as to elongate 
or contract the axis of the eye ; just as a tele- 
scope can be drawn out or shortened, in order 
to adapt it to the distance of the object to be 
viewed. T|ie whole eye-ball is surrounded by 
strong muscles which are capable of effecting 
these requisite changes of distance between the 
cornea and the retina. The Dolphin^which lives 
more constantly in the water, has an eye still 
more nearly approaching in its structure to that 
of fishes; the crystalline lens being nearly 
spherical, and the globe of the eye furnished 
with strong and numer^ous muscles. In birds 
which frequently plunge their heads under 
water the crystalline lens is more convex than 
in other tribes ; and the same is true also of 
aquatic reptiles. 


Chapter VII. 


The object of nature in establishing the organ- 
izations we have been reviewing is to produce 
certain modified impressions on the extremities 
of particular nervous filaments provided to 
receive them ; but these impressions constitute 
only the commencement of the series of cor- 
poreal changes which terminate in sensation; 
for they have to be conveyed along the course 
of the nerves to the brain, or central organ of 
the nervous system,* where, again, some phy- 
sical change must take place, before the re- 
sulting afiection of the mind can be produced. 
The particular part of the brain where this last 
physical change, immediately preceding the 
mental change, takes place, is termed the Sen- 
sorium. Abundant proofs exist that all the 
physical changes here referred to really occur, 

* It 18 usual to designate the end of the neire which is next 
to the sensorium, as the origin of that nerve ; whereas it should 
more properiy be regarded as its termination ; for the series of 
changes which end in sensation commence at the organ of sense, 
and' are thence propagated along the nerve to the sensorium. 


and also that they occur in this order of suc- 
cession : for they are invariably found to be 
dependent on the healthy state, not only of the 
nerve,, but also of the brain; thus, the destruc- 
tion, or even compression of the nerve, in any 
part of its course between the external organ 
and the sensorium, totally prevents sensation ; 
and the like result ensues from even the slight- 
est pressure made on the sensorium itself. 

Although the corporeal or physical change 
taking place in the sensorium, and the mental 
affection we term sensation, are linked together 
by some inscrutable bond of connexion, they 
are, in their nature, as perfectly distinct as the 
subjects in which they occur ; that is, as mind 
is distinct from matter ; and they cannot, there- 
fore, be conceived by us as having the slightest 
resemblance the one to the other. Yet sen- 
sations invariably suggest to the mind ideas, 
not only of the existence of an external agent 
as producing them, but also of various qualities 
and attributes belonging to these agents; and 
the belief, or rather the irresistible conviction, 
thus forced upon us, of the reality of these 
external agents, which we conceive as consti- 
tuting the material world, is termed Perception. 

Various questions here present themselves 
concerning the origin, the formation, and the 
laws of our perceptions. This vast field of 
curious but difiicult inquiry, situated on the 


omfines of the two great departments of huoian 
knowledge, (of which the one relates to the 
phenomena of matter, and the other to those 
of mind,) requires for its succesefbl cultlTatioQ 
the combined efforts of the j^ysiologist and 
the metaphysician. For although our seraa- 
tions are purely mental affections, yet inasmuch 
as they are immediately dependent on physical 
causes, they are reflated by the physical laws 
of the living frame; whereas the perceptions 
derived from these sensations, being the results 
of intellectual processes, are amenable rathar 
to the laws which regulate mental titan physical 
phenomena. It is certain, from innumerable 
facts, that in the present state of our existencei 
the operations of the mind are conducted by the 
instrumentality of our bodily organs; and thai 
unless the brain be in a healthy condition^ these 
operations become disordered, ot altogether 
cease. As the eye and the ear are the instrsH 
ments by which we see and hear, so the braia 
is the material instrument by which we retrace 
and combine ideas, and by which we renember^ 
we reason, we invent. Sudden pressure on thai 
organ, as in a stroke of apoplexy, puts a t^ta) 
stop to all these operations of the mind* If the 
pressure be of a nature to admit of remedy, 
and ha« not inj«red the te^iture of the brain, 
recovery may take place ; and immediately on 
the return of consciousness, the person awaked 


as from a dream, ha^iDg no sense of the time 
which has elapsed since the moment of the 
attack. All causes which disturb the healthy 
condition of the brain, such as alcohol, opium, 
and other narcotic drugs, or which disorder 
more especially the circulation in that organ, 
such as those inducing fever, or inflammation^ 
produce corresponding derangements of the in- 
tellectual powers ; modiiying the laws of the 
association of ideas, introducing confusion in 
the perceptions, irregularity in the trains of 
thought, and incapacity of reasoning, and lead- 
ing to the infinitely diversified forms of mental 
hallucination, delirium, or insanity. Even the 
strongest minds are subject to vicissitudes 
arising fi*om slighter caus«, which affect the 
general tone of the nervous system. Vain, 
indeed, was the boast of the ancient StcHcs 
that the human mind is independent of the 
body, and imp^ietrable to external influences. 
No mortal man, whatever may be the vigour 
of his intellect, or the energy of his application, 
can withstand the influence of impressions on 
his external senses ; for, if i^ifficiently reiterated 
or intense, they will always have power, if not 
to engross his whole attention, at least to in- 
terrupt the current of bis thoughts, and direct 
them into other channels. Kor is it necessary 
for producing this effect that cannon should 
thunder in his ears; the mere rattling of a 


window, or the creaking of a hinge will often 
be sufficient to disturb his philosophical medi- 
tations, and dissever the whole chain of his 
ideas. ^* Marvel not/' says Pascal, '' that this 
profound statesman is just now incapable of 
reasoning justly ; for behold, a fly is buzzing 
round his head. If you wish to restore to 
him the power of correct thinking, and of dis- 
tinguishing truth from falsehood, you must 
first chase away the insect, holding in thraldcmi 
that exalted reason, and that gigantic intellect, 
which govern empires and decide the destinies 
of mankind." 

Although we must necessarily infer, from the 
evidence furnished by experience, that some 
physical changes in the brain accompany the 
mental processes of thought, we are in utter ig- 
norance of the nature of those actions ; and all 
our knowledge on tl^is subject is limited to the 
changes which we are conscious are going on in 
the mind. It is to these mental changes, there- 
fore, that our attention is now to be directed. 

In experiencing mere sensations, whatever be 
their assemblage or order of succession, the mind 
is wholly passive : on the other hand, the mind 
is active on all occasions when we combine into 
one idea sensations of different kinds, (such as 
those which are derived from each separate 
sense), when we compare sensations or ideas with 
one another, when we analyze a compound idea. 


and unite its elements in an order or mode of 
combination different from that in which they 
were originally presented . Many of these active 
operations of mind are implied in the process of 
perception ; for although it might be supposed 
that the diversity in the nature of our sensations 
would suf&ciently indicate to us a corresponding 
variety . in the qualities of the material agents^ 
which produce their impressions on our senses, 
yet these very qualities, nay, even the existence 
of the objects themselves, are merely inferences 
deduced by our reasoning powers, and not the 
immediate effects of those impressions on the 
mind. We talk, for instance, of seeing a distant 
l^y ; yet the immediate object of our perception 
can only be the light, which has produced that 
particular impression on our retina ; whence we 
infer, by a mental process, the existence, the 
position, and the magnitude of that body. When 
we hear a distant sound, the immediate object of 
our perception is neither the sounding body 
whence it emanates, nor the sucqessive undula* 
tions of the medium conveying the effect to our 
ear ; but it is the peculiar impression made by 
the vibrating particles of the fluid, which are in 
direct contact with the auditory nerve. It is 
not difficult to prove that the objects of percep- 
tion are mere creations of the mind, suggested, 
probably instinctively, by the accompanying 
sensations, but having no real resemblance or 



correspondence either with the imprenioM 
themdelves, or with the ageneieft whiish produce 
them ; for many are the iuatances in which oar 
actual perceptions are widely different fram the 
truth, and have no external prototype in natnn. 
In the absence <rfli^t, any mechanical pressiu^ 
suddenly applied to the eye, excites, by its efiect 
on the retina, the sensation c£ vivid light That 
this sensation is present in the mind we are cer- 
tain, because we are conscious of its existenee : 
here there can be no fallacy. But the p»cep^ 
tion of light, as a cause of this sensation, b^ng 
inseparably associated with sueh sensation, and 
wholly dependent on it, bxA all 
Mspects, both as to its duration and intensity^ 
with the 'Same circumstances in the sensation^ 
we cannot av^ having the perceptimi as well^as 
the sensation of light : yet it is certain that mk 
light has acted. The error, then, attaehes Co the 
perception ; and its source is to be traced to the 
mental process by which perception is derived 
fiy)m sensation. 

Many other examples miglit be given oi fiedia^ 
eious perceptions, arising itmxx impreisictns made 
in an unusual manner on the nerves of the 
senses. One of the most remarkable is the Wj^ 
peamnce of a fladi of light from the tranamissMa 
of the galvanic influence through the facial 
nerves. If a piece of silver, or of gold, be 
passed as hi^ as possible betweenChe ^qnp^r 


Up and the gums, while at the same time a 
I^ate of ssinc is laid on the tongue, or applied to 
the inside of the cheeks ; and if a communica^ 
tion be then made between the two metals, 
either by bringing them into direct contact, or 
by means of a wire touching both of them at 
the same time, a flash of light is seen by the 
perscw who is the subject of the expi^iment 
This appearsmce is the effect of an impression 
made either on the retina, or on the optic nerve, 
and is analogous to that occasioned by a mecha* 
aical impulse, such as a blow directed to the 
aame part of the nervous system, both being 
phenomena totally independ^it of the presence 
of light. A similar fallacy occurs in the per- 
cepti<m of taste, which arises in the well klioWB 
^periment of placing a piece of zinc end another 
of sihrer, the one on the upper and the other 
en the under surface of the tongue, and making 
tfaem communicate, when . a pungent and disr 
agreeable metallic taste is instantly perceived : 
this happens because the nerves of the tongue, 
being acted upon by the galvanism thus excited, 
communicate the same sensation as that which 
would be occasioned by the actual application 
of sapid bodies to that wgan. Thus it appears 
that crises which are very differrat in their 
nature, may, by acting on the same nerves, 
produce the very same sensation ; and it follows, 
tiierefore, that our sensations cannot be depended 


upon as being always exactly correspondent with 
the qualities of the external agent which excites 

Evidence to the same effect may also be 
gathered from the consideration of the narrow- 
ness of those limits within which all our senses 
are restricted. It requires a certain intensity in 
the agent, whether it be light, or sound, or che- 
mical substances applied to the senses of smell 
or taste, in order to produce the very lowest 
degree of sensation. On the other hand, when 
their intensity exceeds a certain limit, the 
nature of the sensation changes, and becmnes 
one of pain. Of the sensations commonly re- 
ferred to the sense of touch, there are many 
which convey no perception of the cause pro- 
ducing them. Thus a slighter impression than 
that which gives the feeling of resistance pro- 
duces the sensation of itching, which is totally 
different in its kind. The sensation of cold is 
equally positive with that of warmth, and differs 
from it, not in degree merely, but in species ; 
although we know that it is only in its degree 
that the external cause of each of these sensa- 
tions differs. 

The only distinct notions we are capable of 
forming respecting Matter^ are that it consists af 
certain powers of attraction and repulsion, occu- 
pying certain portions of space,. and capaUe of 
moving in space ; and that its parts thereby 


assume different relative positions or configura- 
tions. But of mind, our knowledge is more ex- 
tensive and more precise, because we are con- 
scions of its existence, and of many of its opera- 
tions, which are comprised in the general term 
thought. To assert that thought can be a pro- 
perty of matter, is to extend the meaning of the 
term matter to that with which we cannot per- 
ceive it has any relation. All that we know of 
matter has regard to space : nothing that we 
know of the properties and affections of mind 
has any relation whatsoever to space. 
' A similar incongruity is contained in the pro- 
position that thought is b, function of the brain. 
It is not the brain which thinks, any more than 
it is the eye which sees, though each of these 
material organs is necessary^for the production 
of these respective effects. That which sees and 
which thinks is exclusively the mind ; although 
it is by the instrumentality of its bodily organs 
that these changes take place. Attention to this 
fundamental distinction, which, although obvious 
when explicitly pointed out,- is often lost sight 
of in ordinary discourse, will furnish a key to 
the solution of many questions relating to per- 
ception, which have been considered as difficult 
and embarrassing. 

- The sensations derived from the different 
senses have no resemblance to one another, and 
have, indeed, no property in common, exc^t 


that tbey are felt by the sftme perdpieiit being. 
A edcmr has no sort of resenriblaiice to a sound ; 
nor have either of these any similarity to a& 
odonr, or a taste, or to the sennkiooB of bent, or 
eokl. But the mind, which receives these in- 
eongmouit dements, has the power of giving 
them, as it were, cohesion, of comparing them 
with one another, of miiting them into eombtna*- 
tionS) and of Jorming them into ideas of external 
objects* All that nature presents is an infinite 
huinber of particles^ scattered in difibrent parts 
of space ; but out of these the mind forms indi- 
vidual groups, to which she gives a unity of her 
own creation* 

All our notions of material bodies involve that 
of spacre ; and we derive this fundamental idea 
firwn the peculiar .sensations which attend the 
actions of our voluntary muscles. These actions 
first give us the idea of our own bodies, of its 
various parts, and of their figure and movements; 
and next teach us the positioti, distances, magni* 
tudes, and figures of adjacent objects. Com- 
bined with these ideas are the more immediate 
perceptions of tbuch, arising ftom contact with 
the skin, and especially with the fingers. All 
these perceptions, variously modified, make us 
acquainted with those mechanical properties of 
bodied, which have been regarded by many 
IIS primary or essential qualities. The per- 
ceptions derived from the other senses can only 


add to the fomer th^ ideas of partial, or secou* 
dacy qualities, mch as temperature, the peculiar 
actions which produce taste and SDObell, the sounds 
coaveyed from certain bodies, and lastly their 
visiUe appearances. 

The picture formed on the retina by the re- 
fracting power of the humours of the eye, is the 
soarcB of all the perceptions which belong to the 
s^ise . of vision : but the visible appearances 
which these pictures immediately suggest, iybe4 
taken by themselves, could have given us no 
nctioa of the situation, distances, or magnitudes 
fd the objects they repr^ent ; and it is altogether 
from the experience acquired by the exercise of 
other senses that we learn the relation which 
these appearances have with those objects. In 
process of time the former become the signs and 
symbols of the latter; while abstractedly^ and 
without such reference, they have no meanings 
The knowledge of these relations is acquired by 
a process exactly analogous to that by which we 
learn a new language. On hearing a certain 
soond in constant coi^junction with a certain idea, 
the two became insepambly associated together 
tn omr minds ; so that on hearing the name, the 
oomsponding idea immediati^ly presents itself. 
In like manner, the visible appearance of an 
object is the sign, which ioitantly impresses us 
witli ideas of the presence, distance^ situation* 
fionn, and dimensions of the body that gave rise 


to it. This association is, in man at least, not 
original, but acquired. The objects of sight and 
touch, as Bishop Berkeley has justly observed, 
constitute two worlds, which although they haye 
a very important correspondence and connexion, 
yet bear no sort of resemblance to one another. 
The tangible world has three dimensions, 
namely, length, breadth, and thickness; the 
visible world only two, namely, length and 
breadth. The objects of sight constitute a kind 
of language, which Nature addresses to our eyes, 
and by which she conveys information most im*- 
portant to our welfare. As, in any language, the 
words or sounds bear no resemblance to the 
things th6y denote, so in this particular language 
the visible objects bear no sort of resemblance to 
the tangible objects they represent. 

The theory of Berkeley received complete 
confirmation by the circumstances attending the 
well known case, described by Cheselden, of a 
boy, who, from being blind from birth, suddenly 
acquired, at the age of twelve, the power of see- 
ing, by the removal of a cataract. He at fiist 
imagined that all the objects he saw touched his 
eyes, as what he felt did his skin ; and he was 
unable either to estimate distances by the sight 
alone, or even to distinguish one object from 
another, until he had compared the visual with 
what has been called the tactual impression. 

This theory also affords a satisfactory solution 


of a question which has frequently been sup- 
posed to invoWe considerable difficulty ; namely, 
how it happens that we see objects in their true 
situation, when their images on the retina, by 
which we see them, are inverted. To expect 
that the impression from an inverted image on 
the retina should produce the perception of a 
similar position in the object viewed, is to com* 
mit the error of mistaking these images for the 
real objects of perception, whereas they are only 
the means which suggest the true perceptions. 
It is not the eye which sees; it is the mind. The 
analogy which the optical part of the eye bears 
to a camera obscura has perhaps contributed to 
the fallacy in question ; for, in using that instru- 
inent, we really contemplate the image which is 
received on the paper, and reflected from it to our 
eyes. But in our own vision nothing of this kind 
takes place. Far from there being any contem- 
plation by the mind of the image on the retina, 
we are utterly unconscious that such an image 
exists, and still less can we be sensible of the 
position of the image with respect to the object. 
All that we can distinguish as to the locality of 
the visual appearance which an object produces, 
is that this appearance occupies a certain place 
in the field of vision ; and we are taught, by the 
experience of our other senses, that this is a sign 
of the existence of the external object in a parti- 
cnlar direction with reference to our own bddy. 


It 18 not until long after tim aseoeiatMm hm 
been established that we learn, by deduction 
from scientific principles, that the part of the 
retina, on which the impression causing this 
appearance is made, is on the side opposite to 
that of the object itself ; and also that the image 
of a straight object is curved as well as inverted. 
But this subsequent informati<m can never in* 
terfere with our habitual, and perhaps instinc- 
tive reference of the appearance resulting from 
an impression made upon the upper part of 
the retina, to an object situated below us, and 
vice vertA, Hence we at once refer impressioiid 
made on any particular part of the retina to a 
cause proceeding from the opposite side. Thus 
if we press the eye-ball with the finger applied 
at the outer comer of the orbit, the luminous 
appearance excited by the pressure is imme- 
diately referred to the opposite or inner side of 
the eye. 

If we place a card perpendicularly between 
the two eyes, and close to the face* the card will 
appear double, because, although each surfiEiee is 
seen by the eye which is adjacent to it, in the 
direction in which it really is with regard to that 
^ye> y^t, being out of the limits of distinct viswn, 
it is r^rred to a much greater distance than 
its real situation ; and consequaitly, the two sides 
of the object appear sepwated by a wide intarvtti, 
and iu9k if th^ir belonged to two differmit objects. 


Many other examples might he given of iimilstf 
feltacies in our visual perceptions. 

All impressions made on the nerves of sensa- 
tion have a definite duration, and continue fer a 
certain interval of time after the adtion of tibe' 
external agent has ceased. The operation of this 
low is most conspicuous in those cases where the 
presence or absence of the agent can readily be 
determined. Thus we retain the sensation of a 
sound for some time after the vibrations of the 
external medium have ceased; as is shown by 
the sensation of a mi:»ical note being the result 
of the regular succession of aerial undulations, 
when the impression made by each continues 
during the whole interval between two't^onsecu-^ 
tive vibrations. The impulses of light on the 
retina are unquestionably consecutive, like those 
of sound, but being repeated at still shorter in- 
tervals, give rise to a continuous impression. A 
falniliar instance of the same principle occurs in 
the appearance of an entire luminous circle, from 
the rapid whirling round of a piece of lighted 
charcoal ; for the part of the retina which re- 
ceives the brilliant image of the burning char- 
coal, retains the impression with nearly the same 
intensity during the entire revolution of the 
light, wh^n the same impression is renewed. 
For the same reason a rocket, or a fiery meteor, 
shooting across the sky in the night, appears to 
leave behind it a long luminous train. The 


exact time during which these impressions con- 
tinue, after the exciting cause has been with- 
drawn, has been variously estimated by different 
experimentalists, and is very much influenced 
indeed, by the intensity of the impression.* 

When the impressions are very vivid, anoth^ 
phenomenon often takes place; namely, their 
subsequent recurrence, after a certain intenral, 

* Mauy curious visual illusions may be traced to the ope- 
ration of this principle. One of the most remarkable is the 
curved appearance of the spokes of a carriage wheel rolling on 
the ground, when iriewed through the intervals between vertical 
parallel bars, such as those of a palisade, or Venetian window- 
blind. On studying the circumstances of this phenomenon I 
found that it was the necessary result of the tracesr left on 
the retina by the parts of each spoke which became in succession 
visible through the apertures, and assumed the curved ap- 
pearances in question. A paper, in which I gave an account of 
the. details of these observations, and of the. theory by which I ex- 
plained them, was presented to the Royal Society, and publislied 
in the Philosophical Transactions, for 1825, p. 131. About 
three years ago, Mr. Faraday prosecuted the subject with the 
usual success which attends all his philosophical researches, 
and devised a great number of interesting experiments on the 
appearances resulting from combinations of revolving wheels ; 
the details of which are given in a paper contained in the fifst 
volume of the Journal of the Royal Institution of Great Britain, 
p. 205. This again directed my attention to the subject, and led 
me to the invention of the instrument which has since been intro- 
duced into notice under the name of the Phantasmascope or 
Phenakisticope. I constructed several of these at that peiiod, 
(in the spring of 1831) which I showed to many of my friends; 
but in consequence of occupations and cares of a more serious 
kind, I did not publish any account of this invention, which 
was last year reproduced on the continent. 


during which they are not felt, and quite in- 
dependently of any renewed application of the 
cause which had originally excited them. If, 
for example, we look steadfastly at the sun for 
a second or two, and then immediately close our 
eyes, the image or spectrum of the sun remains 
for a long . time present to the mind, as if its 
light were still acting on the retina. It then 
gradually fades and disappears ; but if we con- 
tinue to keep the eyes shut, the same impression 
will, after a certain time, recur, and again vanish ; 
and this phenomenon will be repeated at inter- 
vals, the sensation becoming fainter at each re- 
newal. It is probable that these reappearances 
of the image, after the light which produced the 
original impression has been withdrawn, are oc- 
casioned by spontaneous affections of the retina 
itself, which are conveyed to the sensorium. 
In other . cases, where the impressions are 
less strong, the physical changes producing 
these spectra are perhaps confined to the senso- 
rium. These spectral appearances generally 
undergo various changes of colour, assuming first 
a yellow tint, passing then to a green, and lastly 
becoming blue, before they finally disappear. 

Another general law of sensation is, that all 
impressions made on the nerves of sense tend to 
exhaust their sensibility, so that the continued or 
renewed action of the same external cause pro- 
duces a less effect than at first : while, on the 


Other hand, the absence or diminution of the 
t»aal excitement leads to a gradual increase of 
sensibility, so that the subsiequ^it application of 
an exciting cause produces more than the usual 
effect One of the most obvious exemi^ifica- 
tions of this law presents itself in the case of the 
sensations of temperature. The very same body 
may appear warm to the touch at one time, and 
cold at another, (although its real temperature hia 
not varied,) according to the state of the oigaa 
induced by previous impressions: and a very 
different judgment will be formed of its tempe- 
rature, when felt by each hand in succe9)»pB, 
if the one has immediately befcyre be^i exposed 
to cold, while the other has retained its natural 
warmth. Similar phenomena may be observed 
with regard to all the other senses: thus the 
flavour of odorous, as well as sapid btnlies, de- 
pends much on the previous state of the oigan 
by which they are perceived ; any strong imr- 
pressicm of taste nmde on the nerves of the 
tongue, rendering them, for some time, nearly 
insensible to weaker tastes. Sounds, which 
make a powerful impression on the auditory 
nerves, will, in like manner, occasion tenkporary 
deafness with regard to faint sounds. The con- 
verse of this is observed when hearing has been 
suddenly restored in deaf persons, by the opera- 
tion of perforating the ear-drum.* The s^nsi- 

* See the note in p. 434 of this volume. 


bility of the ^auditory nerveB^ which had not 
been accessible to impressions oi iK>uBd, is 
found to be increased to a morbid degree. This 
was remarkably exemplified in the case of ja 
g^iideman who for several years had been ray 
deaf, in consequence of the oUitoation of the 
eustachian tube, so that he could scarcely hear 
a parson speaking in a loud voice close to his 
ear. As soon as the instrument which liad 
BHide the perforation was withdrawn, the by- 
seders began to address hiin in a very low 
tone of voice, and were surprised at receiving no 
answer, and at his remaining immoveable in his 
chair, as if stunned by a vic^ent blow. At 
length he burst out into the exclamation, '* For 
God's sake, gentlemen, refrain from crying out so 
terribly loud I you are giving me exoessive pam by 
speaking to me/' The surgeon,* upon this, re- 
tired across the room; unfortunatdy, however, 
the creaking of his boots caused the gentleman to 
start up in an agony from his chair, at the same 
time ai^lying his hand instinctively to cov^ his 
ear ; but in doing this» the sound of his fiagers 
coming in contact with his head was a fresh 
source of pain, producing an effect similar to 
that of a pistol suddenly fired close to him. For 
along time after, when spd^en to, even in the 
lowest whisper, he complained of the distressing 

* M. Maunoir, of Geneva, on vhose authority I have given 
this account. 


loudness of the sounds : and it was several weeks 
before this excessive sensibility of the auditory 
nerves wore off: by degrees, however, they ac- 
commodated themselves to their proper function, 
and became adapted to the ordinary impressions 
of sound. Some time afterwards, this gentleman 
had a similar operation performed on the other ear, 
and with precisely the same results ; the same 
degree of excessive sensibility to sounds was ma- 
nifested on the restoration of hearing in this ear 
as had occurred in the first ; and an equal tioie 
elapsed before it was brought into its natural 

The most striking illustrations of the extent 
of this law are furnished by the sense of vision. 
On entering a dark chamber, after having been 
for some time exposed to the glare of a bright 
sunshine, we feel as if we were blind ; for the 
retina, having been exhausted by the action of a 
strong, light, is insensible to the weaker impres- 
sions which it then receives. It might be sup^ 
posed that the contraction of the pupil, which 
takes place on exposure to a strong light, and, of 
course, greatly reduces the quantity admitted to 
the retina, is a cause adequate to account for 
this phenomenon : but careful observation will 
show that the pupil very rapidly enlai^es to its 
full expansion when not acted upon by light; 
while the insensibility of the retina continues 
for a much longer time. It regains its usual 


sensibility, indeed, only by slow degrees. By 
remaining in the dark its sensibility is still 
farther increased, and a faint light will ex- 
cite impressions equal to those produced in 
the ordinary state of the eye by a much stronger 
light ; and while it is in this state, the sudden 
exposure to the light of day produces a dazzling 
and painful sensation. 

This law of vision was usefully applied by Sir 
William Herschel in training his eye to the 
acquisition of extraordinary sensibility, for the 
purpose of observing very faint celestial objects. 
It often happened to him, when, in a fine winter's 
night, and in the absence of the moon, he was 
occupied during four, five, or six hours in taking 
sweeps of the heavens with his telescope, that, 
by excluding from the eye the light of surround- 
ing objects, by means of a black hood, the sen- 
sibility of the retina was so much increased, that 
when a star of the third magnitude approached 
the field of view, he found it necessary imme- 
diately to withdraw his eye, in order to preserve 
its powers. He relates that on one occasion the 
appearance of Sirius announced itself in the 
field of the telescope like the^dawn of the morn- 
ing, increasing by degrees in brightness, till the 
star at last presented itself with all the splendour 
of the rising sun, obliging him quickly to re- 
treat from the beautiful but overpowering spec- 



The peculiar construction of the organ of 
vision allows of our distinguishing the eflfects of 
impressions made on particular parts of the 
retina from those made on the rest, and from 
their general eflfect on the whole surface. These 
partial variations of sensibility in the retina give 
rise to the phenomena of ocular spectra^ as they 
are called, which were first noticed by Buffon^ 
and afterwards more fully investigated by Dr, 
Robert Darwin. A white object on a dark 
ground, after being viewed steadfastly till the 
eye has become fatigued, produces, when the eye 
is immediately directed to another field of view, 
a spectrum of a darker colour than the surround- 
ing space, in consequence of the exhaustion 
of that portion of the retina on which its image 
had been impressed. The converse takes place, 
when the eye, after having been steadfastly 
directed to a black object on a light ground, 
is transferred to another part of the same field ; 
and in this case a bright spectrum of the object 
is seen. 

It is a still more curious fact that the sensi- 
bility of the retina to any particular kind of 
light, may, in like manner, be increased or 
diminished, without any change taking plaoe 
in its sensibility to other kinds of light. 
Hence the spectrum of a red object appears 
green ; because the sensibility of that portion of 
the retina, on which the red image has been im- 
pressed, is impaired with regard to the red rays. 

OCULAR Spectra. 531 

while the yellow and the blue rays still continue 
to produce their usual effect ; and these, by com- 
bining their influence, produce the impression of 
green. For a similar reason, the spectrum of a 
green object is red; the rays of that colour 
being those which alone retain their power of fully 
impressing the retina, previously rendered less 
sensible to the yellow and the blue rays com- 
posing the green light it had received from the 
object viewed. 

The judgments we form of the colours of 
bodies are influenced, in a considerable degree, 
by the vicinity of other coloured objects, which 
modify the general sensibility of the retina* 
When a white or grey object of small dimen* 
sions, for instance, is viewed on a coloured 
ground, it generally appears to assume a tint of 
the colour which is complementary to that of 
the ground itself.^ It is the etiquette among the 
Chinese, in all their epistles of ceremony, to 
employ paper of a bright scarlet hue : and I am 
informed by Sir George Staunton, that for a long 
time after his arrival in China, the characters 
written on this kind of paper appeared to him to 
be green ; and that he was afterwards much sur- 
prised at discovering that the ink employed was 
a pure black, without any tinge of colour, and on 
closer examination he found that the marks were 

* Any two colours which, when combined together, produce 
white light, are said to be complementary to one another. 


also black. The green appearance of the letters, 
in this case, was an optical illusion, arising from 
the tendency of the retina, which had been 
strongly impressed with red light, to receive im- 
pressions corresponding to the complementary 
colour, which is green. 

A philosophical history of the illusions of the 
senses would afford ample evidence that limits 
have been intentionally assigned to our powers 
of perception ; but the subject is much too ex- 
tensive to be treated at length in the present 
work.* I must content myself with remarking, 
that these illusions are the direct consequences 
of the very same laws, which, in ordinary cir- 
cumstances, direct our judgment correctly, but 
are then acting under unusual or irregular com- 
binations of circumstances. These illusions 
may be arranged under three classes, according 
as they are dependent on causes of a physical, 
physiological, or mental kind. 

The first class includes those illusions in 
which an impression is really made on the 
organ of sense by an external cause, but in a 
way to which we have not been accustomed. 
To this class belong the acoustic deceptions 
arising from echoes, and from the art of ven- 

* In the Gulstonian Lectures, which I was appointed to read 
to the Royal College of Physicians, in May, 1832, 1 took occa- 
sion to enlarge on this subject. A summary of these lectures 
was given in the London Medical Gazette, vol. x. p 273. 


triloquism ; the deceptive appearances of the 
mirage of the desert, the looming of the hori* 
zon at sea, the Fata Morgana of the coast of 
Calabria, the gigantic spectre of the Brocken in 
the Hartz, the suspended images of concave 
mirrors, the visions of the phantasmagoria, the 
symmetrical reduplications of objects in the 
field of the kaleidoscope, and a multitude of 
other results of the simple combinations of the 
laws of optics. 

The second class comprehends those in which 
the cause of deception is more internal, and 
consists in the peculiar condition of the nervous 
surface receiving the impressions. Ocular spec- 
tra of various kinds, impressions on the tongue 
and the eye from galvanism, and those which 
occasion singing in the ears, arising generally 
from an excited circulation, are among the 
many perceptions which rank under this head. 

The third class of fallacies comprehends those 
which are essentially mental in their origin, and 
are the consequences of errors in our reasoning 
powers. Some of these have already been 
pointed out with regard to the perceptions of 
vision and of hearing, the formation of which is 
regulated by the laws of the association of ideas. 
But even the sense of touch, which has befen 
generally regarded as the least liable to fallacy, 
is not exempt from this source of error, aJ9 is. 
proved by the well known experiment of feeling 


a single l)all, of about the size of a pea, between 
two fingers which are crossed ; for there is then 
a distinct perception of the presence of two 
balls instead of one. 

But limited as our senses are in their range 
of perception, and liable to occasional error, we 
cannot but perceive, that, both in ourselyes, and 
also in every class of animals, they have been 
studiously adjusted, not only to the properties 
and the constitution of the material world, bnt 
also to the respective wants and necessities of 
each species, in the situations and circumstances 
where it has been placed by the gracious and 
beneficent Author of its being. 

If the sensorial functions had been limited to 
mere sensation and perception, conjoined with 
the capacity of passive enjoyment and of suf- 
fering, the purposes of animal existence wouM 
have been but imperfectly accomplished ; for in 
order that the sentient being may secure the 
possession of those objects which are agreeable 
and salutary, and avoid or reject those which 
are painful or injurious, it is necessary that he 
possess the power of spontaneous action. Hence 
the faculty of Voluntafy Motion is superadded 
to the other sensorial functions. The muscles 
w^ich move the limbs, the trunk, the head, and 
organs of sense, — all those parts, in a word, 
which establish relations with the external 
world, are, through the intermedium of a sepa- 
rate set of nervous filaments, totally distinct from 


those which are subservient to sensation,* made 
to communicate directly with the sensorium, and 
are thereby placed under the direct control and 
guidance of the wilL The mental act of volition 
is doubtless accompanied by some corresponding 
' physical change in that part of the sensorium, 
whence the motor nerves^ or those distributed to 
the muscles of voluntary motion, arise. Here, 
then, we pass from mental phenomena to such 
as are purely physical; and the impression, 
whatever may be its nature, originating in the 
fiensorium, is propagated along the course of the 
nerve to those muscles, whose contraction is re- 
quired for the production of the intended action. 
Of the function of voluntary motion, as &r as 
concerns the moving powers and the mechanism 
of the instruments employed,! I have already 

* On this subject I must refer the reader to the researches of 
Sir Charles Bell, and Magendie, who have completely established 
the distinction between these two classes of nerves. 

t A voluntary action, occurring as the immediate consequence 
of the application of an external agent to an organ of the senses, 
though apparently a simple phenomenon, implies the occurrence 
of no less than twelve successive processes, as may be seen 
by the following enumeration. First, there is the modifying 
action of the organ of the sense, the refractions of the rays, for 
instance, in the case of the eye : secondly, the impression made 
on the extremity of the nerve : thirdly, the propagation of this 
impression along the nerve : fourthly, the impression or physical 
change in the sensorium. Next follow four kinds of mental 
processes, namely, sensation, perception, association, and volition. 
Then, again, there is another physical change taking place in the 
sensorium, immediately consequent on the mental act of volition : 
this is followed by the propagation of the impression downwards 


treated at sufficient length in the first part of 
this work* 

Every excitement of the sensorial powers is, 
sooner or later, followed by a proportional de- 
gree of exhaustion ; and when this has reached 
a certain point, a suspension of the exercise of 
these faculties takes place, constituting the 
state of sleep y during which, by the continued 
renovating action of the vital functions, these 
powers are recruited, and rendered again adequate 
to the purposes for which they were bestowed. 
In the ordinary state of sleep, however, the ex- 
haustion of the sensorium is seldom so complete 
as to preclude its being excited by internal 
causes of irritation, which would be scarcely 
sensible during our waking hours: and hence 
arise dreams, which are trains of ideas, sug^ 
gested by internal irritations, and which the 
mind is bereft of the power to control, in con- 
sequence of the absence of all impressions from 
the external senses.^ In many animals, a much 
more general suspension of the actions of life, 
extending even to the vital functions of respi- 
ration and circulation, takes place during the 
winter months, constituting what is termed 

along the motor nerve; then an impression is made on the 
muscle; and lastly we obtain the contraction of the muscle, 
which is the object of the whole series of operations. 

* The only indications of dreaming given by the lower animals 
occur in those possessed of the greatest intellectual powers, such 
as the Dofff among quadrupeds, and the Parrot ^ among birds. 



Chapter VIII. 



§ 1 • Nervous Systems of Invei^tehrated Animals. 

Our knowledge of the exact uses and functions 
of the various parts which compose the nervous 
system, and especially of its central masses, is 
unfortunately too scanty to enable us to discern 
the correspondence, which undoubtedly exists, 
between the variations in the functions and the 
diversities in the organization. The rapid re- 
view which I propose to take of the different 
plans, according to which the nervous system is 
constructed in the several classes of animals, 
will show that these central masses are multi- 
plied and developed in proportion as the facul- 
ties of the animal embrace a wider range of 
objects, and are carried to higher degrees of 

In none of the lowest tribes of Zoophytes, 
such as Sponges^ Polypi^ and Medusa^ have any 
traces of organs, bearing the least analogy to a 
nervous system, been discovered ; not even in 


the largest specimens of the last named tribe, 
some of which are nearly two feet in diameter. 
All these animals give but very obscure indica- 
tions of sensibility; for the contractions they 
exhibit, when stimulated, appear to be rather 
the eflfect of a vital property of irritability than 
the result of any sensorial fac^ty. Analogy, 
however, would lead, us to the belief that many 
of their actions are really prompted by sensa- 
tions and volitions, though in a degree very 
inferior to those of animals higher in the scale of 
being : but whatever may be their extent, it h 
probable that the sensorial operationis in these 
animals take place without the intervention of 
any common sensorium, or centre of action. It 
is at the same time remarkable that their 
movements are not effected by means of mus- 
cular fibres, as they are in all other ani- 
mcds, the granular flesh, of which their whole 
body is composed, appearing to have a generally 
diffused irritability, and perhaps also some de- 
gree of sensibility ; so that each isolated granule 
may be supposed to be endowed with these com- 
bined properties, performing, independ^itly of 
the other granules, the functions both of narve 
and muscle. Such a mode of existence exhibits 
apparently the lowest and most rudimental con- 
dition of the animal functions. Yet the actions 
of the Hydra, of which I have given an account, 
are indicative of distinct volitions ; as are also, in 


a still more decided manner, those of the Infu- 
soria. In the way in which the latter avoid ob« 
stacles while swimming in the fluid, and turn 
aside when they encounter one another, and in 
the eagerness with which they pursue their prey, 
we can hardly fail to recognise the evidence of 
voluntary action. 

To seek for an elucidation of these mysteries 
in the structure of animals whose minuteness 
precludes all accurate examination, would be a 
hopeless inquiry. Yet the indefatigable Ehren^ 
berg has recently discovered, in some of the 
lai^er species of animalcules belonging to the 
tnrder Roti/era, an organization, which he be* 
lieves to be a nervous system. He observed, in 
the Hydatina senta^ a series of six or seven grey 
bodies, enveloping the upper or dorsal part of 
the oesophagus, closely connected together, and 
perfecdy distinguishable, by their peculiar tint, 
from the viscera and the surrounding parts. 
The uppermost of these bodies, which he con- 
siders as a ganglion, is much larger than the 
others, and gives off slender nerves, which, by 
joining another ganglion, situated under the in- 
teguments at the back of the neck, form a circle 
of nerves, analogous to that which surrounds the 
oesophagus in the mollusca : from this cirde two 
slender nervous filaments are sent off to the 
head, and a larger branch to the abdominal sur- 
face of the body. • The discovery of a regular 


structure of muscular bands of fibres, in these 
animalcules, is a further evidence of the con- 
nexion which exists between nerves and muscles. 

We again meet with traces of nervous fila- i 
ments, accompanied also with muscular bands of 
fibres, in some of the more highly oi^anized 
Entozoa. In the Ascarisj or long round worm, 
a slender and apparently single filament is seen 
passing forwards, along the lower side of the 
abdomen, till it reaches the cesophagus, where it 
splits into two branches, one passing on each 
side of that tube, but without exhilHting any 
ganglionic enlargement. This may be consi- 
dered as the first step towards the particular 
form of the nervous system of the higher classes 
of articulated animals, where the principal ner- 
vous cord is obviously double throughout its 
whole length, or, if partially united at different 
points, it is always readily divisible into two, by 
careful manipulation. In addition to this cha- 
racteristic feature, these cords present in their 
course a series of enlargements, appearing like 
knots ; one pair of these generally corresponding 
to each of the segments of the body, and sending 
ofi*, as from a centre, branches in various direc- 
tions. It is probable that these knots, or ganglia, 
perform, in each segment of the worm, an office 
analogous to that of the brain and special mar- 
row of vertebrated animals, serving as centres of 
nervous, and perhaps also of sensorial powers. 


Many facts^ indeed, tend to show that each 
segment of the body of articulated animals, of 
an annular structure and cyliridric form, such as 
the long worms and the myriapoda, has in many 
respects an independent sensitire existence, so 
that when the body is divided into two or more 
parts, each portion retains both the faculty of 
sensation, and the power of voluntary motion < 
As far as we can judge, however, the only ex- 
ternal sense which is capable of being exercised 
by this simple form of nervous system, is that of 
touch ; all the higher senses evidently requiring 
a much more developed and concentrated orga- 
nization of nervous ganglia. 

In this division of the animal kingdom, the 
primary nervous cords always pass along the 
middle of the lower surface of the body, this 
being the situation which, in the absence of a 
vertebral bony column, affords them the best 
protection. They may be considered as ana* 
logous to the spinal marrow, and as serving to 
unite the series of ganglia, through which they 
pass, into one connected system. On arriving 
at the oesophagus, they form round it a circle, or 
collar, studded with ganglia, of which the up- 
permost, or that nearest the head, is generally of 
greater size than the rest, and is termed the 
cesophagealj cephalic^ or cerebral ganglion, being 
usually regarded as analogous to the brain of 
larger animals. Perhaps a more correct view of 


its functions wonld be conveyed by calling it the 
principal brain, and considering the other ganglia 
as subordinate brains. This large ganglion, which 
supplies an abundance of nervous filaments to 
every part of the head, seems to be the chief 
organ of the higher senses of vision, of hearing, 
of taste, and of smell, and to be instrumental in 
combining their impressions, so as to constitute 
an individual percipient animal, endowed with 
those active powers which are suited to its rank 
in the scale of being. 

Such is the general form of the nervous system 
in all the Annelida : but in the higher orders of 
Articulata we find it exhibiting various degrees 
of concentration. The progress of this concen- 
tration is most distinctly traced in the Crustacea,^ 
One of the simplest forms of these organs occurs 
in a little animal of this class, which is often 
found in immense numbers, spread over tracts of 
sand on the sea shore, and which is called the 
^^® ^«««<2B,^ Talitrus locusta, or Sand-hopper, 

(Fig. 438). The central parts 
of its nervous system are seen 
in Fig. 439, which represents the abdominal side 
of this animal laid open, and magnified to twice 
the natural size. The two primary nervous 
cords, which run in a longitudinal direction, are 

* See the account of the researches of Victor Audouin, and 
H. M. Edwards, on this subject, given in the Ann. des Sc. Nat. 
xix. 181. 


here perfectly distinct from one another, and 
even separated by a small interval : they present 
a series of ganglia, which are nearly of equal 
size, and equidistant from one another, one pair 
correspcmding to each segment of the body,* 
and united by transverse threads: and other 
filaments, diverging laterally, proceed from each 
ganglion. During the progress of growth, the 
longitudinal cords approach somewhat nearer to 
each other, but still remain perfectly distinct. 

The first pair of ganglia, or the cephalic, have 
beeu. considered, though improperly, as the 
brain of the animal. 

The next step in the gradation occurs in the 

* These B^meots are numbered in this uid the following 
figure iD their proper order, beginning with that near tbe head. 
A is the external antenna ; a, the internal antenna ; and e, the 


Phyllosama (Leach), where the ganglia compos- 
ing each pair in the abdomen and in the head, 
are united into single masses, while those in the 
thoracic region are still double. In the CfM- 
thoa (Fab.)y which belongs to the family of 
OniscuSy there is the appearance of a single chain 
of ganglia, those on the one side having coa* 
lesced with those on the other ; each pair com- 
posing a single ganglion, situated in the middle 
line ; while the longitudinal cords which comiect 
them still remain double, as is shown in Fig. 440, 
which represents the interior of this crustaceoos 
animal, nearly of the natural size. But in the 
higher orders of Crustacea, as in the Lobster, 
these longitudinal corda are themselves united in 
the abdominal region, though still distinct in the 

In following the ascending series of crustace- 
ous animals, we observe also an approximation 
of the remoter ganglia towards those near the 
centre of the body : this tendency already shows 
itself in the shortening of the hinder part of the 
nervous system of the Cymothoay as compared 
with the Talitt'us; and the concentrati<xi proceeds 
farther in other tribes. In the Palemon^ for 
example, most of the thoracic ganglia, and in the 
Palinurus (Fab.), all of them, have coalesced 
into one large oval mass, perforated in the 
middle, and occupying the centre of the thorax ; 


and lastly, in the Mma squinado, or Spider 
Oab (Fig. 441),* this mass acquires still greater 
compactness, assumes a more globular form, and 
has no central perforation. 

These different forms of structure are also 
exemplified in the progress of the developement 

* In this figure are seen the great thoracic ganglion (b), from 
which proceed the superior thoracic nerves (t), those to the 
foie feet (f), to the hinder feet (f), and the abdominal nervous 
trunk (n) ; the cephalic ganglion (c), communicating by means 
of two nervous cords (o), which surround the cesophagus and 
entrance into the stomach (s), with the thoracic ganglion (b) ; 
and sending off the optic nerve (e) to the eyes (b), and the motor 
nerves (m), to the muscles of those organs ; and also the nerves 
(a) to the internal antennee, and the nerves (x) to the external 
anlennte (a). 

VOL. 11. N N 


of the higher Crustacea : thus, in the Lobster, the 
early condition of the nervous system is that of 
two separate parallel cords, each having a dis- 
tinct chain of ganglia, as is the case in the Tali- 
trus : then the cords are observed gradually lo 
approximate, and the ganglia on each side to 
coalesce, as represented in the Cymothoa: and 
at the period when the limbs b^in to be deve- 
loped, the thoracic ganglia approach one ano- 
ther, unite in clusters, and acquire a rapid en- 
largement, preparatory to the growth of the 
extremities from that division of the body, the 
abdominal ganglia remaining of the same size as 
before. The cephalic ganglion, which was ori- 
ginally double, and has coalesced into one, is 
also greatly developed, in correspondence with 
the growth of the organs of sense. The next 
remarkable change is that taking place in the 
hinder portions of the nervous cords, which are 
shortened, at the same time that their ganglia 
are collected into larger masses, preparatory to 
the growth of the tail and hinder feet ; so that 
throughout the whole extent of the system the 
number of ganglia diminishes in the prepress of 
developement, while their size is augmented. 

All Insects have the nervous system con- 
structed on the same general model as in the 
last mentioned classes ; and it assumes, as in the 
Crustacea, various degrees of concentration in 
the different stages of developement. As an 




example we may take the nervous system of the 
Sphinx ligtistriy of which representations are 
given in the larva, pupa, and imago states, 
wholly detached from the body, and of their 
natural size, in Figures 442, 443, and 444.* 






* Tliese figures were drawn by Mr. Newport, from original 
preparations made by himself. The same numbers in each refer 
to the same parts ; so that by comparing the figures with one 
another, a judgment may be formed of the changes of size and 
situation which occur in the progress of the principal transfor- 
mations of the insect. Numbers 1 to 1 1 indicate the series of 
ganglia which are situated along the under side of the body, and 
beneath the alimentary canal. Of these the first ^ye are the 
thoracic, and the last six the abdominal ganglia ; while the ce- 


This system in the larva (Fig. 442) has the 
same simple form as in the Annelida, or in the 

phaltc, or cerebral ganglion (17) is situated alxnre the oesophagus 
and dorsal yessel, and communicates by two nervous coids with 
the first of the series, or sub-oesophageal ganglion (1), which is, in 
every stage of the insect, contained within the head, and distri- 
butes nerves to the parts about the mouth. The next gangtioo 
(2) becomes obliterated at a late period of the change {rom the 
pupa to the imago state : the third (3) remains, but the two 
next (4, 5) coalesce to form, in the imago, the large thoracic 
ganglion ; while the two which follow (6 and 7), become vhoUy 
obliterated before the insect attains the imago state, the interven- 
ing cords becoming shorter, and being, with the nerves they send 
out, carried forwards. The last four (8, 9, 10, 11) of the abdo- 
minal ganglia remain, with but little alteration, in all the stages 
of metamorphosis : in the larva, they supply nerves to the felse 
feet. The nerves (12, 13) which supply the wings of the imago, 
are very small in the larva; and they arise by two roots, onederived 
from the cord, and one from the ganglion. The nerves sent to 
the three pair of anterior, or true legs, are marked 14, 15, 16. 

The nervous system of the larva is exhibited in Fig. 442, that 
of the pupa in Fig. 443, and that of the imago in Fig. 444. It 
will be seen that in the pupa the abdominal ganglia are but little 
changed ; but those situated more forward (6, 7) are brought 
closer together by the shortening of the intervening cord, prepa- 
ratory to their final obliteration in the imago ; a change which 
those in front of them (4, 5) have already undergone. The pro- 
gressive developement of the optic (18) and antennal (19) nerves 
may also be traced. Mr. Newport has also traced a set of nerva 
(20) which arise from distinct roots, and which he found to be 
constantly distributed to the organs of respiration. 

A detailed account of the anatomy of the nervous system of the 
Sphinx ligustri, and of the changes it undergoes up to a certain 
period, is given by Mr. Newport in a paper in the Phil. Trans, for 
1832, p. 383. He has since completed the inquiry to the last 
transformation of this and other insects, and has lately presented 
to the Royal Society an account of his researches. 


TalitruSy for it consists of a longitudinal series o^ 
ganglia, usually twelve or thirteen in numbery 
connected in their whole length by a double 
filament. By degrees the different parts of 
which it consists approach each other, the tho- 
racic ganglia, in particular, coalescing into 
larger masses, and becoming less numerous, 
some being apparently obliterated; the whole 
cord becomes in consequence shorter, and the 
abdominal ganglia are carried forwards. The 
optic nerves are greatly enlarged during the 
latter stages of transformation, and are often 
each of greater magnitude than the brain itself. 
A set of nerves has also been discovered, the 
course of which is peculiar, and appears to cor- 
respond with the sympathetic or ganglionic sys- 
tem .of nerves in vertebrated animals, while 
another nerve resembles in its mode of distri- 
bution, the pneumo' gastric nerve, or par vagum. 
Very recently Mr. Newport has distinctly traced 
a separate nervous tract, which he conceives 
gives origin to the motor nerves, while the 
subjacent column sends out the nerves of sen- 

In the next great division of the animal king- 
dom, which includes all molluscous animals, 
the nervous ganglia have a circular, instead of 
a longitudinal arrangement. The first example 
of this type occurs in the Asteriasj where the 
nervous system (Fig. 446) is composed of small 



ganglia, equal in number to the rays of the 
, animal, and disposed in a circle round the cen- 
tral aperture or mouth, but occupying situations 
intermediate between each of the rays. A nerve 
is sent off from both sides of each ganglion, and 
passes along the side of the rays, each ray 
receiving a pair of these nerves. In the Hoto- 
thuria there is a similar chain of ganglia, 

encircling the cesophagus ; and the same mode 
of arrangement prevails in all the biTalve M<d- 
lusca, except that, besides the (esophageal ganglia, 
others are met with, in different parts of the 
body, distributing branches to the -viscera, and 
connected with one another and Mdth the oeso- 
phageal ganglia by filaments, so as to form with 
them one continuous nervous system. In the 


Gasteropoda, which are furnished with a distinct 
head, and organs of the higher senses, (such as 
the Aplysia^ of which the nervous system is ex- 
hibited in Fig. 446), there is generally a special 
cephalic ganglion (c), which may be supposed to 
serve the office of brain.* In others, again, as 
in the Patella (Fig. 447), the cephalic ganglion 
is scarcely discernible, and its place is supplied 
by two lateral ganglia (l, l) ; and there is be- 
sides a transverse ganglion (t), below the ceso- 
phagus. The cephalic ganglion, on the other 
hand, attains a considerable size in the Cepha- 
lopoda (c, Fig. 448), where it has extensive con- 
nexions with all the parts of the head : the 
optic ganglia (o, o), in particular, are of very 
great size, each of them, singly, being larger 
than the brain itself.f 


* This figure also shows a ganglion (a), which is placed higher, 
and communicates by lateral filaments with the cephalic ganglion 
(c) ; two lateral ganglia (l, l), of great size ; and a large abdo- 
minal ganglion (g). 

t Some peculiarities in the structure of the cephalic ganglion 
of the Sepia have been supposed to indicate an approach to the 
vertebrated structure ; for this ganglion, together with the laby- 
rinth of the ear, is enclosed in a cartilaginous ring, perforated at 
the centre to allow of the passage of the oesophagus, and imagined 
to be analogous to a cranium. 





D ^ 

15 w 









§ 2. Nervous System of Vertehrated Animals. 

The characteristic type of the nervous system of 
vertebrated animals is that of an elongated cy- 
linder of nervous matter (m z, Fig. 449), ex- 
tending down the back, and lodged in the canal 
formed by the grooves and arches of the verte- 
brae. It has received the name of spinal marrow, 
or more properly ^ma/ cord: and, (as is seen in 
the transverse section, Fig. 450), is composed of 
six parallel columns, two posterior, two middle, 
and two anterior, closely joined together, but 
leaving frequently a central canal, which is filled 
with fluid. On each side of the spinal cord, and 
between all the adjacent vertebrae, there proceed 
two sets of nervous filaments, those which are 
continuous with the posterior columns (p), being 
appropriated to the fimction of sensation; and 
those arising from the anterior columns (a), being 
subservient to voluntary motion. The former, 
soon after their exit from the spine, pass through 
a small ganglion (g), and then unite with the 
nerves from the anterior column, composing, by 
the intermixture of their fibres, a single nerv- 
ous trunk (n), which is afterwards divided and 
subdivided in the course of its farther distribu- 
tion, both to the muscular and the sentient 
organs of the body. Each of these spinal nerves 
also sends branches to the ganglia of the sympa- 


thetic nerve, which, as was formerly described, 
passes down on each side, parallel and near 
to the spine. 

Enlargements of the spinal marrow are ob- 
served in those parts (w and l. Fig. 449), which 
supply the nerves of the extremities, the increase 
of diameter being proportional to the size of the 
limbs requiring these nerves. In Serpents, 
which are wholly destitute of limbs, the spinal 
marrow is not enlarged in any part, but is a 
cylindrical column of uniform diameter. In 
Fishes, these enlargements are in proportion to 
the relative size and muscularity of the lateral 
fins, and correspond to them in their situation. 
The Piper Gurnard {Trigla lyra)y which is a 
species of flying fish, having very large pectoral 
fins, that portion of the spinal marrow supplying 
their muscles with nerves (as seen in the space 
between m and s, Fig. 451), has numerous en- 
largements, presenting a double row of tubercles. 
Fishes which possess electrical organs have a 
considerable dilatation of the spinal marrow, 
answering to the large nerves which are dis- 
tributed to those organs. Birds which fly but 
imperfectly, as the Gallinaceotis tribe and the 
Scansoresj have the posterior enlargement much 
greater than the anterior ; a disproportion which 
is particularly remarkable in the Ostrich. On 
the contrary, the anterior enlargement is much 
more considerable than the posterior in birds 
which have great power of flight. In the Dove, 


of which the brain and whole extent of the 
spinal marrow are shown in Fig. 449, the en- 
largements (w and l) corresponding to the wings 
and leffs respectively, are nearly of equal size. 
lo Qu^pJL, we likewise find tl,e iltive size 
of these enlargements corresponding to that of 
fore and hind extremities. When the latter are 
absent, as in the Cetacean the posterior dilatation 
does not exist. 

The brain (b) may be r^arded as an expan- 
sion of the anterior or upper end of the spinal 
miarrow ; and its magnitude, as well as the 
relative size of its several parts, vary much 
in the different classes and families of ver- 
tebrated animals. This will appear from the 
inspection of the figures I have given of this 
oi^an in various species, selected as specimens 
£rom each class, viewed from above ; and in all 
of which I have indicated corresponding parts 
by the same letters of reference. 

The portion (m) of the brain, which appears 
as the immediate continuation of the spinal 
marrow (s), is termed the medulla oblongata. 
The single tubercle (c), arising from the ex- 
pansion of the posterior columns of the spinal 
marrow, is termed the cerebellum^ or little brain. 
Next follow the pair (t) which are termed the 
optic tuberclesy or lohes^* and appear to be pro- 

• In the Mammalia, and in Man, they have been often desig- 
nated by the very inappropriate name of Corpora quadri- 


ductions from the middle columns of the spinal 
marrow. These are succeeded by another pair 
of tubercles (h), which are called the cerebral 
hemispheres^ and the origin of which may be 
traced to the anterior columns of the spinal 
marrow. There is also generally found, in front 
of the hemispheres, another pair of tubercles (o), 
which, being connected with the nerves of smell- 
ing, have been called the olfactory hhesy or 
tubercles.* These are the principal parts of the 
cerebral mass to be here noticed; for I pur- 
posely omit the mention of the*minuter divisions, 
which, though they have been objects of much 
attention to anatomists, unfortunately furnish no 
assistance in understanding the physiology of 
this complicated and wonderful organ. 

On comparing the relative proportions of the 
brain and of the spinal marrow in the four 
classes of vertebrated animals, a progressive in- 
crease in the size of the former will be observed 
as we ascend from Fishes to Reptiles, Birds, 
and Mammalia. This increase in the magnitude 
of the brain arises chiefly from the enlargement 
of the cerebral hemispheres (h), which in the 
inferior orders of fishes, as in the Trigla lyra^ or 
Piper Gurnard (Fig. 451), and in the Murtena 
conger, or Conger Eel (Fig. 452), are scarcely 

• Several cavities, termed Ventricles^ are occasionally found 
in the interior of the principal tubercles of the brain ; but their 
use is unknown. 


discernible. They are very small in the Pei^ca 
Jluviatilisy or common Perch (Fig. 453); but 
more developed in Reptiles, as in the Testudo 
mydasy or Green Turtle (Fig. 454), and in the 
Crocodile (Fig. 455) ; and still more so in Birds, as 
is seen in the brain of the Dove (Fig. 449) ; but 
most of all in Mammalia, as is exemplified in 
the brain of the Lion (Fig. 456). On the other 
hand, the optic tubercles (t) are largest, com- 
pared with the rest of the brain, in Fishes ; and 
their relative size diminishes as we ascend to 
Mammalia: aud the same observation applies 
also to the olfactory lobes (o). 

The relative positions of the parts of the brain 
are much influenced by their proportional deve- 
lopement. This will be rendered manifest by 
the lateral views of the brains of the Perch, the 
Turtle, the Dove, and the Lion, presented in 
Figures 457, 458, 459, and 460, respectively, 
where the same letters are employed to designate 
the same parts as in the preceding figures. In 
Fishes, all the tubercles which compose this 
organ, are disposed nearly in a straight line, 
continuous with the spinal marrow, of which, as 
they scarcely exceed it in diameter, they appear 
to be mere enlargements. As the skull expands 
more considerably than the brain, this organ 
does not fill its cavity, but leaves a large space, 
filled with fluid. Some degree of shortening, 
however, may be perceived in the brain of the 


Perch (Fig. 457) ; for the medulla oblongata (m) 
is doubled underneath the cerebellum (c), pul- 
ing it upwards, and rendering it more prominent 
than the other tubercles. This folding inwards, 
and shortening of the whole mass, proceeds to a 
greater extent as we trace the structure upwards, 
as may be seen in the brain of the Green Turtle 
(Fig. 458). In that of Burds^ of which Fig. 459 
presents a vertical section, the optic tubercles 
have descended from their former place, and 
assumed a lateral position, near the lower sur- 
face of the brain, lying on each side of the 
medulla oblongata, at the part indicated by the 
letter t. In Mammalia, as in the Lion (Fig. 
460), they are lodged quite in the interior of the 
organ, and concealed by the expanded hemi- 
spheres (h) ; their position only being marked 
by the same letter (t). These changes are con- 
sequences of the increasing developement of the 
brain, compared with that of the cavity in which 
it is contained, requiring every part to be more 
closely packed ; thus the layers of the hemi- 
spheres in Mammalia are obliged, from their 
great extent, to be plaited and folded on 
one another, presenting at the surface curious 
windings, or canvolutiansy as they are called 
(seen in Fig. 456), which do not take place 
in the hemispheres of the inferior classes. The 
foldings of the substance of the cerebellum pro- 
duce, likewise, even in birds, transverse furrows 


on the surface ; and from the interposition of a 
substance of a grey colour between the laminee 
of the white medullary matter, a section of the 
cerebellum presents the curious appearance 
(seen in Fig. 459), denominated, from its fancied 
resemblance to a tree, the Arhor Vitie. 

Thus far we have followed an obvious gradation 
in the developementand concentration of the dif- 
ferent parts of the brain : but on arriving at Man, 
the continuity of the series is suddenly disturbed 
by the great expansion of the hemispheres, 
(Fig. 461), which, compared with those of quad- 
nipeds, bear no sort of proportion to the rest of 
the nervous system. Both Aristotle and Pliny 
have asserted that the absolute, as well as the 
comparative size of the human brain is greater 
than in any other known animal : exceptions, 
however, occur in the case of the Elephant j and 
also in that of iAxeWhale^ whose brains are certainly 
of greater absolute bulk than that of man. But 
all the large animals, with which we are familiarly 
acquainted, have brains considerably smaller; as 
will readily appear from an examination of their 
skulls, which are narrow and compressed at the 
part occupied by the brain ; the greater part of 
the head being taken up by the developement of 
the face and jaws. In Man, on the other hand, 
the bones of the skull rise perpendicularly from 
the forehead, and are extended on each side, so 
as to form a capacious globular cavity for the 


receptioQ and defence of this most important 
oi^an. It is chiefly from the expansion of the 
hemispheres, and the derelopement of its codto- 
lutions, that the human brain derives this great 
augmentation of size.* 

* This will be apparent from the vertical section of the haroan 
brain, Pig. 461 ; where, a> before, s is the spinal marrow; u, 
the medulla oblongata; c, the cerebellum, with the arbor vUte; 
T, the optic tubercles, or corpora quadrigemina, dwindled to a 
very small size, compared with their bulk in fishes ; p, the 
pineal gland, supposed by Des Cartes to be the seat of the 
soul; V, one of the lateral ventricles; q, the corpus callosum; 
and H, R, H, the hemispheres. 

Several expedients have been proposed for estimating the 
relative size of the brain in different tribes of animals, with a 
view of deducing conclusions as to the constancy of the relatirai 
which is presumed to exist between its greater magnitude and 
the possession of higher intellectual faculties. The most cele- 
brated is that devised by Camper, and which he termed the 
facial angle, composed of two lines, one drawn in the direction 


§ 3. Functions of the Brain. 

Physiologists have in all ages sought for an 
elucidation of the functions of the brain by the 
accurate examination of its structure, which 
evidently consists of a congeries of medullary 
fibres, arranged in the most intricate manner. 
Great pains have been bestowed in unravelling 
the tissue of these fibres, in the hope of dis- 
covering some clue to the perplexing labyrinth of 
its organization: but nearly all that has been 
learned from the laborious inquiry, is that the 
fibres of the brain are continuous with those 
which compose the columns of the spinal 
marrow ; that they pass, in their course, through 
masses of nervous matter, which appear to be 
analogous to ganglia ; and that their remote 
extremities extend to the surface of the convo- 
lutions of the brain and cerebellum, which are 
composed of a softer and more transparent grey 
matter, termed the cortical or cineritious sub- 
stance of the brain. 

of the basis of the skull, from the ear to the roots of the upper 
incisor teeth, and the other from the latter point, touching 
the most projecting part of the forehead. Camper conceived 
that the magnitude of th^s angle would correctly indicate the 
size of the brain, as compared with the organs of the principal 
senses which compose the face : but the fallacy of this criterion 
of animal sagacity has been shown in a great many cases. 



It is a remarkable fact, that in vertebrated 
animals all the organs which are subservient to 
the sensorial fiinctions are double, those on one 
side being exactly similar to those on the other. 
We see this in the eyes, the ears, the limbs, and 
all the other instruments of voluntary motion ; 
and in like manner the parts of the nervous 
system which are connected with these functions 
are all double, and arranged symmetrically <m 
the two sides of the body. The same law of 
symmetry extends to the brain: every part <^ 
that organ which is found on one side is repeated 
on the other ; so that, strictly speaking, we have 
two brains, as well as two optic nerves and two 
eyes. But in order that the two sets of fibres 
may co-operate, and constitute a single organ of 
sensation, corresponding with our consciousness 
of individuality, it was necessary that a free 
communication should be established between 
the parts on both sides. For this purpose there 
is provided a set of medullary fibres, passing 
directly across from one side of the brain to the 
other ; these constitute what are called the Cam- 
missures of the Brain.* 

* The principal commissure of the human brain, called tlie 
corpus callosumf is seen at q, Fig. 461. Dr. Macartney, in a 
paper which he read at the late meeting at Cambridge of the 
British Association for the Advancement of Science, described 
the structure of the human brain, as discovered by his pacnlwr 
mode of dissection, to be much more complicated than is 


The question, however, still recurs; — What 
relation does all this artificial intertexture and 
accumulation of fibres bear to the mental ope- 
rations of which we are conscious, such as 
memory, abstraction, judgment, imagination, 
Toliti<m? Are there localities set apart for our 
different ideas in the store-house of the cerebral 
hemispheres, and are they associated by the 
matmal channels of communicating fibres? 
Are the mental phenomena the effSects, as was 
formerly supposed, of a subtle fluid, or animal 
spirits, circulating with great velocity along 
invisible canals in the nervous substance? or 
shall we, with Hartley, suppose them to be the 
results of vibrations and vibratiuncles, agitating 
in succession the finer threads of which this 
mystic web has been constructed ? But a little 
reflection will suffice to convince us that these, 
and all other mechanical hypotheses, which the 
most fancifiil imagination can devise, make not 
the smallest approach to a solution of the diffi- 
culty ; for they, in fact, do not touch the real 
subject to be explained, namely, how the affec- 
tions of a material substance can influence and 
be influenced by an immaterial agent. AU that 

genen^j supposed. He observed that its fibces are interlaced 
in the n^ost intricate manner, resembling the plexuses met with 
among the nerves, and establishing the most extensive and 
general communications between every part of the cerebral 


we have been able to accompliah has been to 
trace the impressions from the organ of sense 
along the communicating nerve to the aen- 
sorium : beyond this the clue is lost, and we can 
fpUow the process no farther. 

The exact locality of the sensorium has been 
eagerly sought for by physiologists in every age. 
It would appear, from the results of the most 
recent inquiries, that it certainly does not extend 
to the whole mass of the brain, but has its seat 
more especially in the lower part, or basis of 
that organ. It differs, however, in its locality, 
in different classes of animals. In man, and 
the mammalia which approach the nearest to 
him in their structure, it occupies some part of 
the region of the medulla oblongata, probably 
the spot where most of the nerves of sense are 
observed to terminate. In the lower animals it 
is not confined to this region, but extends to the 
upper part of the spinal marrow. As we de- 
scend to the inferior orders of the animal king- 
dom, we find it more and more extensively dif- 
fused over the spinal marrow; and in the In- 
vertebrata the several ganglia appear to be 
endowed with this sensorial property : but, 
becoming less and less concentrated in single 
masses, the character of individuality ceases to 
attach to the sensorial phenomena; until, in 
Zoophytes, we lose all traces of ganglia and of 
nervous filaments, and every part appears to 


possess an inherent power of exciting sensation, 
as well as performing muscular contractions. 

Beyond this point we can derive no further 
aid from Anatomy, since the intellectual ope- 
rations of which we are conscious bear no con- 
ceivable analogy with any of the configurations 
or actions of a material substance. Although 
the brain is constructed with evident design, and 
composed of a number of curiously wrought 
parts, we are utterly unable to penetrate the 
intention with which they are formed, or to 
perceive the slightest correspondence which 
their configuration can have with the functions 
they respectively perform. The map of regions 
which modem Phrenologists have traced on the 
surface of the head, and which they suppose to 
have a relation to different faculties and pro- 
pensities, does not agree either with the natural 
divisions of the brain or with the metaphysical 
classification of mental phenomena.* Experi- 
ments and pathological observations, however, 
seem to show that the hemispheres of the brain 
are the chief instruments by which the intel- 
lectual operations are carried or ; that the 
central parts, such as the optic lobes and the 

^ For a summary of tbe doctrines of Drs. Gall and Spurzheim, 
I beg leave to refer the reader to an account which I drew up, 
many years ago, for the EBcyclopcedia Britannica, and which 
composed the article '' Cranioscopt *' in the last supplement 
to Uiat work, edited by Mr. Napier. 


medulla oblongata, are those principally con- 
cerned in sensation ; and that the cerebelluin is 
the chief sensorial agent in voluntary motion. 

^ 4« Comparative Physiology of Perceptiam. 

Of the perceptions of the lower animals, and of 
the laws which they obey, our knowledge must, oi 
necessity, be extremely imperfect, since it must 
be derived from a comparison with the results of 
our own sensitive powers,' which may differ very 
essentially from those of the subjects of our 
observation. The same kind of organ which, in 
ourselves, conveys certain definite feelings, may, 
when modified in other animals, be the^ source 
of very different kinds of sensations and per- 
ceptions, of which our minds have not the power 
to form any adequate conc^tion. Many of the 
qualities of surrounding bodies, which escape 
our more obtuse senses, may be distinctly per- 
ceived, in all their gradations, by particular 
tribes of animals, furnished with more delicate 
organs. Many quadrupeds and birds possess 
powers of vision incomparably more extensive 
than our own ; in acuteness of hearing, we are 
excelled by a great number of animals, and in 
delicacy of taste and smell, there are few quad- 
rupeds which do not far surpass us. The organ 


of smell, in particular, is often spread over a 
vast extent of surface, in a cavity occupying the 
greatest part of the head ; so that the per- 
ceptions of this sense must be infinitely diver- 

Bats have been supposed to possess a peculiar, 
or sixth sense, enabling them to perceive the 
situations of external objects without the aid 
either of vision or of touch. The principal facts 
upon which this opinion has been founded were 
discovered by SpaUanzani, who observed that 
these animals would fly about rapidly in the 
darkest chambers, although various obstacles 
were purposely placed in their way, without 
striking against or even touching them. They 
continued their flight with the same precision as 
before, threading their way through the most 
intricate passages, when their eyes were com- 
pletely covered, or even destroyed. Mr. Jurine, 
who made many experiments on these animals, 
concludes that neither the senses of touch, of 
hearing, or of smell, were the media through 
which bats obtain perceptions of the presence 
and situation of surrounding bodies; but he 
ascribes this extraordinary faculty to the great 
sensibility of the skin of the upper jaw, mouth, 
and external ear, which are fomished with very 
large nerves.* 

* Sir Anthony Carlisle attributes this .power to the extreme 
delicacy of hearing in this animal. 


The wonderfiil acuteness and power of dis- 
crimination which many animals exercise in the 
discovery and sdiecticm of their food, has ofiea 
suggested the existence of new s^Qses, different 
from those which we possess, and conveyiDg 
peculiar and unknown powers of p»ceptioD* 
An organ, which appears to perfonn some sen- 
sitive function of this kind, has been discovered 
in a great number of quadrupeds by Jacobson.* 
In the human skeleton there exists a small per- 
foration in the roof of the mouth, just behind the 
sockets of the incisor teeth, forming a communi- 
cation with the under and fore part of the nos- 
trils. This canal is perceptible only in the dried 
bones ; for, in the living body, it is completely 
closed by the membrane lining the mouth, which 
sends a prolongatiom into it : but in quadrupeds, 
this passage is pervious, even during life, and is 
sometimes of considerable width. Jacobaon 
found, on examining this structure with attas- 
tion, that the canal led to two glandular organs 
of an oblong shape, and enclosed in carti- 
laginous tubes : each gland has in its centre a 
cavity, which communicates above with the 
general cavity of the nostrils. These organs lie 
concealed in a hollow groove within the bone, 
where they are carefully protected from injury: 
and they receive a great number of nerves and 

* See Annates du Mus4e; xviii. 412. 


blood-vessels, resembling in this respect the 
organs of the senses. Their structure is the 
same in all quadrupeds in which they have been 
examined ; but they are largest in the family of 
the Rodentia^ and next in that of the Rttminantia : 
in the Horse, they are still very large, but the 
duct is not pervious ; while in carnivorous quad- 
rupeds, they are on a smaller scale. In Mon- 
keysy they may still be traced, although ex- 
tremely small, appearing to form a link in the 
chain of gradation connecting this tribe with the 
human race, in whom every vestige of these 
organs has disappeared, excepting the aperture 
in thfe bones already noticed. Any use that can 
be attributed to these singularly constructed 
organs must evidently be quite conjectural. The 
ample supply of nerves which they receive 
would indicate their performing some sensitive 
function ; and their situation would point them 
out as fitting them for the appreciation of objects 
presented to the mouth to be used as food : 
hence it is probable that the perceptions they 
convey have a close affinity with those of smell 
and taste. 

The larger cartilaginous fishes, as Sharks and 
RaySj have been supposed by Treviranus to be 
endowed with a peculiar sense, from their having 
an organ of a tubulax structure on the top of the 
head, and immediately under the skin; Roux 
considers it as conveying sensations intermediate 


between those of touch and hearing ; while De 
Blainville and Jacobson regard it merely as the 
organ of a finer touch. 

The perceptive powers of Insects must em* 
brace a very different^ and, in many respects, 
more extended sphere than our own. These 
animals manifest by their actions that they per- 
ceive and anticipate atmospheric changes, of 
which our senses give us no information. It is 
evident, indeed, that the impressions made by 
external objects on their sentient organs must be 
of a nature widely different from those which the 
same objects communicate to ourselves. While 
with r^ard to distance and magnitude our per* 
ceptions take the widest range, and appear infi- 
nitely extended when compared with those of 
insects, yet they may, in other respects, be 
greatly inferior. The delicate discrimination of 
the more subtle affections of matter is perhaps 
compatible only with a minute scale of organi- 
zation. Thus the varying degrees of moisture 
or dryness of the atmosphere, the continual 
changes in its pressure, the fluctuations in its 
electrical state, and various other physical con- 
ditions, may be objects of distinct perception to 
these minute animals. Organs may exist in 
them, appropriated to receive impressions, of 
which we can have no idea ; and opening 
avenues to various kinds of knowledge, to which 


we must ever remain utter straiigers. Art, it is 
true, has supplied us with instruments for dis- 
covering and measuring many of the properties 
of matter, which our unassisted senses are in- 
adequate to observe. But neither our ther- 
mometers, nor our electroscopes, our hygro- 
meters, nor our galvanometers, however skilfully 
devised or elaborately constructed, can vie in 
delicacy and perfection with that refined appa- 
ratus of the seises, which nature has bestowed 
on the minutest insect. There is reason to 
believe, as Dr. WoUaston has shown, that the 
hearing of insects comprehends a range of per- 
ceptions very different from that of the same 
sense in the larger animals ; and that a class of 
vibrations too rapid to excite our auditory nerves, 
is perfectly audible to them. Sir John Her- 
schel has also very clearly proved that, if we 
admit the truth of the undulatory theory of light, 
it is easy to conceive how the limits of visible 
colour may be established ; for if there be no 
nervous fibres in unison with vibrations more or 
less frequent than certain limits, such vibrations, 
though they reach the retina, will produce no 
sensation. Thus it is perfectly possible that 
insects, and other animals, may be incapable of 
being affected by any of the colours which we 
perceive ; while they may be susceptible of re- 
ceiving distinct luminous impressions from a 


class of vibrations which, applied to our visual 
organs, excite no sensation.* The functions of 
the antennae, which, though of various forma^ 
are organs universally met with in this class 
of animals, must be of great importance, thoagh 
obscurely known ; for insects when deprived of 
them appear to be quite lost and bewildered. 

The Torpedo^ the Crymnotus^ and several 
other fishes, are furnished with an electrical 
apparatus, resembling the Voltaic battery, wjiicb 
they have the power of charging and discharging 
at pleasure. An immense profusion of nerves is 
distributed upon this organ ; and we can hardly 
doubt that they communicate perceptions, with 
regard to electricity, very different from any that 
we can feel. In general, indeed, it may be re- 
marked, that the more an organ of sense differs 
in its structure from those which we ourselves 
possess, the more uncertain must be our know- 
ledge of its ftmctions. We may, without any 
great stretch of fancy, conceive ourselves placed 
in the situation of the beasts of the forest, and 
comprehend what are the feelings and motives 
which animate the quadruped and the bird. 
But how can we transport ourselves, even in 
imagination, into the dark recesses of the ocean, 
which we know are tenanted by multitudinous 
tribes of fishes, zoophytes, and moUusca ? How 

* Encyclopeedia Metropolitana, Article ^^ Light.'* 


can we figure to ourselyes the sensitive exiist- 
ence of the worm or the insect, organized in so 
different a manner to ourselves, and occupying 
so remote a region in the expanse of creation ? 
How can we venture to speculate on the percep- 
tions of the animalcule, whose world is a drop of 
fluid, and whose fleeting existence, chequered 
perhaps by various transformations, is destined 
to run its course in a few hours ? 

Confining our inquiries, then, to the more in- 
telligible intellectual phenomena displayed by 
the higher animals, we readily trace a gradation 
which corresponds with the developement of the 
central nervous organ, or brain. That the com- 
parison maybe fairly made, however, it is neces- 
sary to distinguish those actions which are the 
result of the exercise of the intellectual faculties, 
from those which are called instinctive, and are 
referable to other sources. Innumerable are the 
occasions in which the actions of animals appear 
to be guided by a degree of sagacity not deriv- 
able from experience, and apparently implying 
a fore-knowledge of events, which neither ex* 
perience nor reflection could have led them to 
anticipate. We cannot sufficiently admire the 
provident care displayed by nature in the pre- 
servation both of the individual and of the spe- 
cies, which she has entrusted, not to the slow 
and uncertain calculations of prudence, but to 
innate faculties, prompting, by an unerring im- 


pulse, to the performance of the actions required 
for those ends. We see animals protidmg 
against the approach of wint^, the effects ai 
which they have never experienced, and em- 
ploying various means of defence against ene- 
mies they have never seen. The parent consults 
the welfare of the offepring she is destined never 
to behold ; and the young discovers and pursues 
without a guide that species of food which is 
best adapted to its nature. All these unex- 
plained, and perhaps inexplicable facts, we must 
content ourselves with classing under the head of 
instincty a name which ifi, in fact, but the ex- 
pression of our ignorance of the nature <^ that 
agency, c^ which we cannot but admire the 
ultimate effects, while we search in vain for the 
efficient cause. 

In all the inferior orders of the animal crea- 
tion, where instincts are multiplied, while the 
indications of intellect are feeble, the <Nrgaii 
which performs the office of the brain is compa- 
ratively small. The sensitive existence of these 
animals appears to be circumscribed within the 
perceptions of the moment, and their voluntary 
actions have reference chiefly to objects which 
are present to the sense. In proportion as the 
intellectual faculties of animals are multiidied, 
and embrace a wider sphere, additional magni- 
tude and complication of structure are given to 
the nervous substance which is the organ of 


fhoee faculties. The greater the power of com- 
bining ideas, and of retaining them in the me- 
mory, the greater do we find the developement 
of the cerebral hemispheres. These parts of the 
brain are comparatively small, as we have seen, 
in fishes, reptiles, and the greater number of 
birds ; but in the mammalia they are expanded 
in a degree nearly proportional to the extent of 
memory, sagacity, and docility. In man, in 
whom all the faculties of sense and intellect are 
so harmoniously combined, the brain is not only 
the largest in its size, but beyond all comparison 
the most complicated in its structure.'*' 

A large brain has been bestowed on man, evi- 
dently with the design that he should exercise 
superior powers of intellect; the great distin- 
guishing features of which are the capacity for 
retaining an immense variety of impression«; 
and the strength, the extent, and vast range of 
the associating principle, which combines them 
into groups, and forms them into abstract ideas. 
Yet the lower animals also possess their share of 
memory, and of reason : they are capable of 
acquiring knowledge from experience ; and, on 

* All the parts met with in the brain of animals exist also in 
the brain of man ; while several of those found in man are either 
extremely small, or altogether absent in the brains of the lower 
animals. Soemmerring has enumerated no less than fifteen ma- 
terial anatomical differences between the human brain and that 
of the ape. 


some rare occasions, of devising expedients for 
accomplishing particular ends. But still tlus 
knowledge and these efforts of intellect are con- 
fined within very narrow limits ; for nature has 
assigned boundaries to the advancement of the 
lower animals, which they can never pass. If 
one favoured individual be selected for a special 
education, some additional share of inteUigence 
may, perhaps, with infinite pains, be infused; 
but the improvement perishes with that indivi- 
dual, and is wholly lost to the race. By far the 
greater portion of that knowledge which it im- 
ports them to possess is the gift of nature, who 
has wisely implanted such instinctive impulses 
as are necessary for their preservation. Man 
also is born with instincts, but they are few in 
number compared with those of the lower ani- 
mals, and unless cultivated and improved by 
reason and education, would, of themselves, pro- 
duce but. inconsiderable results. That of which 
the effects are most conspicuous, and which is 
the foundation of all that is noble and exalted in 
our nature, is the instinct of Sympatl^. The 
affections of the lower animals, even between 
individuals of the same species, are observable 
only in a few instances : for in general they are 
indifferent to each other's joys or sufferings, and 
regardless of the treatment experienced by their 
companions. The attachment, indeed, of the 
mother to her offspring, as long as its wants and 


feebleness require her aid and protection, is as 
powerful in the lower animals, as in the human 
species : but its duration, in the former case, is 
confined, even in the most social tribes, to the 
period of helplessness; and the animal instinct is 
not Succeeded, as in man, by the continued inter- 
course of affection and kind offices, and those 
endearing relations of kindred, which are the 
sources of the purest happiness of human life. 

While Nature has apparently frowned on the 
birth of man, and brought him into the world 
weak, naked, and defenceless, unprovided with 
the means of subsistence, and exposed on every 
side to destruction, she has in reality implanted 
in him the germ of future greatness. The help* 
lessness of the infant calls forth the fostering 
cure and tenderest affections of the mother, and 
lays the deep foundations of the social union. 
The latent energies of his mind and body are 
successively, though slowly developed. While 
the vital organs are actively engaged in the exe- 
cution of their different offices, while the diges- 
tive apparatus is exercising its powerful chemis- 
try, while myriads of minute arteries, veins, and 
absorbents are indefatigably at work in building 
and modelling this complex frame, the sentient 
principle is no less assiduously and no less inces- 
santly employed. From the earliest dawn of sen- 
sation it is ever busy in arranging, in combining, 
and in strengthening the impressions it receives. 

VOL. II. p p 


W(Hiderful as is the formation of the bodily fabric, 
and difficult as it is to collect its history, still 
more marvellous is the progressive constractum 
of the human mind, and still more arduous the 
task of tracing the finer threads which connect 
the delicate web of its ideas, which fix its fleet- 
ing perceptions, and which establish the vast 
system of its associations, and of following the 
long series of gradations by which its affections 
are expanded, purified, and exalted, and the 
soul prepared for its higher destination in a 
future stage of existence. 

Here, indeed, we perceive a remarkable inter- 
ruption to that r^ular gradation, which we have 
traced in all other parts of the animal series; 
for between man and the most sagacious of the 
brutes there intervenes an immense chasm, of 
which we can hardly estimate the magnituda 
The functions which are purely vital, and are 
necessary for even the lowest degree of sensitive 
existence, are possessed equally by all animals: 
in the distribution of the faculties of mere sen* 
sation a greater inequality may be perceived: 
the intellectual faculties, again, are of a more 
r^ned and nobler character, and being less 
essential to animal life, are dealt out by nature 
with a more sparing and partial hand. Between 
the two extremities of the scale we fi^ld an infi- 
nite number of intermediate degrees. The more 
exalted faculties are possessed exclusively by 


man, and oonstitate the source of the immehse 
superiority he enjoys over the brute creation, 
which so frequently excels him in the perfection 
of subordinate powers. In strength and swift- 
ness he is surpassed by many quadrupeds. In 
vain may he wish for the pow« of flight pos- 
sessed by the numerous inhabitants of air. He 
may envy that range of sight which enables the 
bird to discern, from a height at which it is itself 
invisible to our eyes, the minutest objects on the 
surface of the eattli. He may regret the dulU 
ness of his own senses, when he adviaarts to the 
exquisite scent of the hound, or the acute hear- 
ing of the bat. While the delicate perceptions 
of the lower animals teach them to seek the food 
which is salutary, and avoid that which is inju- 
rious, man alone seems stinted in his powers of 
discrimination, and is compelled to gather in* 
struction from a painful and hazardous expe- 
rience. But if nature has created him thus 
apparently helpless, and denied him those in- 
stincts with which she has so liberally furnished 
the rest of her offspring, it was only to confer 
upon him gifts of infinitely higher value. While 
in acuteness of sense he is surpassed by inferior 
animals, in the powers of intellect he stands 
unrivalled. In the fidelity and tenacity with 
which impressions are retained in his memory, 
in the facility and strength with which they are 
associated, in grasp of comprehension, in extent 


of reasoning, in capacity of progressive improve- 
ment, he leaves all other animals at an immea- 
surable distance behind. He alone enjoys in 
perfection the gift of utterance ; he alone is able 
to clothe his thoughts in words; in him alone 
do we find implanted the desire of examining 
every department of nature, and the power of 
extending his views beyond the confines of this 
globe. On him alone have the high privileges 
been bestowed of recognising and of adoring the 
Power, the Wisdom, and the Goodness of the 
Author of the Universe, from whom his being 
has emanated, to whom he owes all the blessings 
which attend it, and by whom he has been 
taught to look forward to brighter skies and to 
purer and more exalted conditions of existence. 
Heir to this high destination, Man discards all 
alliance with the beasts that perish ; confiding in 
the assurance that the dissolution of his earthly 
frame destroys not the germ of immortality 
which has been implanted within him, and by 
the developement of which the great scheme of 
Providence here commenced, will be carried on, 
in a fiiture state of being, to its final and perfect 



Chapter I. 


Limits have been assigned to the duration of all 
living beings. The same power to whom they 
owe their creation, their organization, and their 
endowments, has also subjected them to the in- 
exorable Law of Mortality; and has ordained 
that the series of actions which characterise the 
state of life, shall continue for a definite period 
only, and shall then terminate. The very same 
causes which, at the earlier stages of their exist* 
ence, promoted their developement and growth, 
and which, at a maturer age, sustained the 
vigour and energies of the system, produce, by 
their continued and silent operation, gradual 
changes in the balance of the functions, and, at 
a later period, effect the slow demolition of the 
fabric they had raised, and the successive de- 
struction of the faculties they had originally 


nurtured and upheld.^ With the germs of life, 
in all organized structures, are conjoined the 
seeds of decay and of death; and however 
great may be the powers of their vitality, we 
know that those powers are finite, and that a 
time mi|st come when they will be expended, 
and when their renewal, in that individual, is no 
longer possible. 

But although the individual perishes. Nature 
has taken special care that the race shall be 
constantly preserved, by providing for the pro- 
duction of new individuals, each springing from 
its predecessor in endless perpetuity. The pro- 
cess by which this formation, or rather this ap- 
parent creation, of a living being is effected, 
surpasses the utmost powers of the human com- 
prehension; No conceivable combinations of 
mechanical, or of chemical powers, bear the 
slightest resemblance, or the most remote ana- 
logy, to organic reproduction, or can afford the 
least clue to the solution of this daik and hope- 
less enigma. We must be content to observe 
and generalize the phenomena, in silent wonder 
at the marvellous manifestation of express con- 
trivance and design, exhibited in this depart- 
ment of the economy of created beings. 

Throughout the whole, both of the y^etabie 

• See the article "Age," id the Cyclopctdia of Practical 
Medicine, where I have enlarged od this subject. 


and animal world, Nature has shown the utmost 
solicitude to secure not only the multiplication 
of the speeies, but also the dissemination of their 
numbers over every habitable and accessible 
region of the globe, and has pursued various 
plans for the accomplishment of these important 

The simplest of all the modes of multiplica- 
tion consists in the spontaneous division of the 
body of the parent into two or more parts ; * each 
part, when separated, becoming a distinct indi- 
vidual, and soon acquiring the size and shape of 
the parent. We meet with frequent examples of 
this process of Jissiparous genefi-ation^ as it is 
termed, among the infusory animalcules. Many 
species of Monads^ iox instance, which are natu- 
rally of a globular shape, exhibit at a certain 
period of their developement a slight circular 
groove round the middle of their bodies, which 
by degrees becoming deeper, changes their form 
to that of an hour-glass; and the middle part 
becoming still more contracted, they present the 
appearance of two balls, united by a mere point. 
The monads in this state are seen swimming 
irregularly in the fluid, as if animated by two 
different volitions ; and, apparently for the pur- 
pose of tearing asunder the last connecting 
fibres, darting through the thickest of the crowd 
of surrounding animalcules; and the moment 
this slender ligament is broken, each is seen 


moving away from the other, and begmning its 
independent existence. This mode of separation 
is illustrated by Fig. 462, representing the suc- 
cessive changes of form during its progress. 

462 a 9 8 3 8%. 


^ \P \7 

In this animalcule the division is transverse, but 
in others, for example in the Vorticella, (as 
shown in Fig. 463), and in most of the larger 
species, the line of separation is longitudinal. 
Each animalcule, thus formed by the subdivision 
of its predecessor, soon grows to the size which 
again determines a further spontaneous subdivi- 
sion into two other animalcules ; these, in coarse 
of time, themselves undergo the same process, 
and so on, to an indefinite extent. The most 
singular circumstance attending this mode of 
multiplication is that it is impossible to pro- 
nounce which of the new individuals thus 
formed out of a single one should be regarded as 
the parent, and which as the offspring, for they 
are both of equal size. Unless, therefore, we 
consider the separation of the parts of the parent 
animal to constitute the close of its individual 
existence, we must recognise an unbroken conti- 


nuity in the vitality of the animal, thus trans- 
mitted in perpetuity from the original stem, 
throughout all succeeding generations. This, 
however, is one of those metaphysical subtleties 
for which the subject of reproduction affords 
abundant scope, but which it would be foreign 
to the object of this work to discuss. 

It is in the animal kingdom only that we 
meet with instances of this spontaneous division 
of an organic being into parts, where each re- 
produces an individual of the same species. All 
plants, however, are capable of being multiplied 
by artificial divisions of this kind : thus a tree 
may be divided longitudinally into a great num- 
ber of portions, or slips^ as they are called, any 
one of which, if planted separately and supplied 
with nourishment, may continue to grow, and 
may, in time, reproduce a tree similar in all 
respects to the one from which it had originated. 
This inherent power of reproduction exists even 
in smaller fragments of a plant ; for, when all 
circumstances are favourable, a stem will shoot 
from the upper end of the fragment, and roots 
will be sent forth from its lower end ; and ulti- 
mately a complete plant will be formed.* These 

* Among the conditions necessary for these evolutions of 
organs are, first, the previous accumulation of a store of nourish- 
ment in the detached fragment, adequate to supply the growth 
of the new parts ; and secondly, the presence of a sufficient quan- 
tity of circulating sap, as a vehicle for the transmission of that 


facts, which are w€ll known to agriculturalists, 
exhibit only the capabilities of y^etatiye power 
under circnmstanoes which do not occur in the 
natural course of things, but have been the effect 
ef human interference. 

Reproductiye powers of a similar kind are 
exhibited very extensively in the lower depart- 
ments of the animal kingdom. The Hydra, or 
fresh water polype, is capable of indefinite mul- 
tiplication by simple division : thus, if it be cut 
asunder transversely, the part containing the 
head soon supplies itself with a tail ; and the 
detached tail soon shoots forth a new head, with 
a new set of tentacula. If any of the tentacnla, 
or any portion of one of them, be cut off, the 
mutilation is soon repaired ; and if the whole 
animal be divided into a great, number of piec^es, 
each fragment acquires, in a short time, all the 
parts which are wanting to render it a complete 
individual. The same phenomena are observed, 
and nearly to the same extent, in the Planaria. 
The AsteriaSy the Actiniuy and some of the lower 
species of Annelida, as the Nats, are also capable 

nourishment. It has been found that when these conditions are 
present, even theleaf of an orange tree, when planted in. a fa- 
vourable soil, sends down roots, and is capable of giving origin 
to an entire tree. According to the observations of Mirandola, 
the leaf of the Bryophyllum, when simply laid on moist ground, 
strikes out roots, which quickly penetrate into the soil. (De 
Candolle, Physiologic V^g^tale^ ii. 677.) The leaves of the mo- 
nocotyledonous plants often present the ^me phcBomenon. 


of being nmltiplied by artificial divisions, each 
segment having the power of supplying others, 
and containing within itself a kind of separate 
and individual vitality. 

A power of more partial regeneration of muti* 
lated parts by new growths, which is very 
analogous to that of complete reproduction, 
exists in the higher orders of animals, though it 
does not extend to the entire formation of two 
individuals out of one. The daws, the feet, and 
the antennee of the Crustacea^ and the limbs of 
the Arachnida^ tire restored, when lost, by a 
fresh growth of these oi^ans. If the head of a 
Snail be amputated, the whole of that part of 
the animal, including the telescopic eyes, and 
other organs of sense, will be reproduced. Even 
among the yertelN*ata we find instances of these 
renovations of mutilated parts ; as happens with 
respect to the fins of fishes: for Broussonet 
found that in whatever direction they are cut, 
the edges easily unite ; and the rays themselves 
are reproduced, provided the smallest part of 
their base has been left. The tails of Newts^ 
and of some species of Lizards, will grow again, 
if lost : and what is more remarkable, the eyes 
themselves, with all their complex apparatus of 
coats and humours, will, if removed, be replaced 
by the growth of new eyes as perfect as the 
former. We have seen that the teeth of Sharks 
and other fishes are renewed with the utcnost 


fisicility, when by accident they have been lost. 
Among Mammalia, similar powers exist, although 
they are restricted within much narrower limits ; 
as is exemplified in the formation of new boneSi 
replacing those which have perished. When 
we advert to the numberless instances of the 
reparation of injuries happening to various parts 
of our own frame, we have abundant reason to 
admire and be grateful for the wise and bountiM 
provisions which Nature has made for meeting 
these contingencies. 

The multiplication of the species by buds, or 
Gemmiparous reproduction^ is exemplified on the 
largest scale in the vegetable creation. Almost 
every point of the surface of a plant appears to 
be capable of giving rise to a new shoot, which, 
when fully developed, exactly resembles the 
parent stock, and may, therefore, be regarded as 
a separate organic being. The origin of buds is 
wholly beyond the sphere of our observation; 
for they arise from portions of matter too minute 
to be cognizable to our organs, with every 
assistance which the most powerful microscopes 
can supply. These imperceptible atoms from 
which organic beings take their rise, are called 

Vegetable germs are of two kinds ; those 
which produce sterns^ and those which produce 
roots: and although both may be evolved from 
every part of the plant, the former are usually 


developed at the axillae of the leaves ; that is, at 
the angles of their junction with the stem ; and 
also at the extremities of the fibres of the stems ; 
their developement being determined by the 
accumulation of nourishment around them. 
They first produce hudsy which expanding, and 
putting forth roots, assume the form of shoots ; 
and the successive accumulation of shoots, which 
remain attached to the parent plant,* and to 
each other, is what constitutes a tree. What 
are called knots in wood are the result of germs, 
which, in consequence of the accumulation of 
nourishment around them, are developed to a 
certain extent, and then cease to grow. The 
Lemna^ or common Duckweed, which consists 
of a small circular leaf, floating on the surface 
of stagnant pools, presents a singular instance 
of the developement of germs from the edges of 
the leaved, and the subsequent separation of the 
new plant thus formed. In this respect the 

* In some rare instances the shoots are removed to a distance 
from the parent plant, by a natural process : this occurs in some 
creeping plants, which propagate themselves by the horizontal 
extension of their branches on the ground, where they dip, and 
strike out new roots, giving rise to stems independent of the 
original plant. This also sometimes happens in the case of 
tuberous roots, as the potatoe, which contaii^ a number of germs, 
surrounded by nutritive matter, ready to be developed when cir- 
cumstances are favourable. These portions are called eyes ; 
and each of them, when planted separately, are readily evolved, 
and give rise to an individual plant. 


process is analogous to the natural mode of mttl- 
tiplication met with in the lower orders of 2^- 
phytes, such as the Hydra. At the ^earliest 
period at which the young of this animal is 
visible, it appears like a small tubercle, or bud, 
rising from the surface of the parent hydra : it 
grows in this situation, and remains attached for 
a considerable period ; at first deriving its non- 
rishment, as well as its mechanical support, 
from the parent ; then occasionally stretching 
forth its tentacula, and learning the art of catch- 
ing and of swallowing its natural prey. The 
tube, which constitutes its stomach, at first com- 
municates by a distinct opening with' that of its 
parent : but this opening afterwards closes ; and 
the filaments by which it is connected with the 
parent becoming more and more slender, at 
length break, and the detached hydra imme- 
diately moves away, and commences its career 
of independent existence. This mode of multi* 
plication, in its first period, corresponds exactly 
with the production of a vegetable by buds ; 
and may therefore be classed among the in- 
stances of gemmiparous reproduction ; although 
at a later stage, it difiers from it in the complete 
detachment of the offspring from the parent. 

Another plan of reproduction is that in which 
the germs are developed in the interior of the 
animal, assuming, at the earliest period when 
they become animated, the form of the parent. 


In this case they are termed gemmules instead 
of huds. This mode of reproduction is exem- 
plified in the Volvox^ which, as we have abready 
seen, is an infusorial animalcule of a spherical 
form, exhibiting incessant revolving move- 
ments.* The germs of this animal are deve- 
loped, in great numbers, in its interior, having 
a globular shape, and visible, by the aid of the 
microscope, through the transparent covering; 
and while yet retained within the body of the 
parent other still minuter globules are developed 
within these, constituting a third generation of 
these animals. After a certain period, the young, 
which have thus beeii formed, escape by the 
bursting of the parent volvox, which in conse- 
quence perishes. Similar phenomena are pre- 
sented by many of the Infusoria. In some of 
the Entozoa, likewise, as in the Hydatid^ the 
young are developed within the parent ; and this 
proceeds successively for an indefinite number 
of generations.! In most cases of the spon- 

* Vol. i. p. 188. This animal ia delineated in Fig. 79. 

i* The mode in which isfusory animalcules are produced and 
maltiplied is involved in much obscurity. Many distinguished 
naturalists, adopting the views of BufTon, have regarded them as 
the product of an inherent power belonging to a certain class of 
material particles, which, in circumstances favourable to its ope- 
ration, tends to form these minute oi^anizations, and in this 
manner they explain how the same organic matter which had 
composed former living aggregates, on the dissolution of their 
union, reappears under new forms of life, and gives rise to the 
phenomenon of innumerable animalcules, starting into being. 


taneous eyolution of gemmides within the parent, 
channels are provided for their exit : but the 
gemmnles of the Actinia force their ¥ray through 
the sides of the body, which readily open to give 

and commencing a new, but fleeting career of existence. Yet 
the analogy of every other department of the animal and vege- 
table kingdoms is directly opposed to the supposition that any 
living being can arise without its having been originally derived 
from*an individual of the same species as itself, and of which it 
once formed a part. The difficulty which the hypothesis of the 
spontaneous production of infusory animalcules professes to 
remove, consists in our inability to trace the pre-existence of the 
germs in the fluid, where these animalcules are found to arise ; 
and to follow the operations of nature in these regions of infinite 
minuteness. The discoveries of Ehrenberg relative to the orga* 
nization of the Rotifera go far towards placing these diminutive 
beings more on a level, both in structure and in functions, with 
the larger animals, of whose history and economy we have a 
more familiar and certain knowledge, and in superseding the 
hypothesis above referred to, by showing that the bold assump- 
tion on which it rests, is not required for the explanation of the 
observed phenomena. In many of these animalcules, he has 
seen the ova excluded in the form of extremely minute globules, 
the 12,000th of an inch in diameter. When these had grown 
to the size of the 1700th of an inch, or seven times their onginai 
diameter, they were distinctly seen to excite currents, and to 
swallow food. The same diligent observer detected the young 
of the Rotifer vulgaris^ perfectly formed, moving in the interior 
of the parent animalcule, and excluded in a living state, thus 
constituting them viviparous animals, as the former were ovi- 
parous. Other species, again, imitate the hydra, in being what 
is termed gemmiparous^ that is, producing gemmules (like the 
budding of a plant), which shoot forth from the side of the 
parent, and are soon provided with cilia, enabling them, when 
separated, to provide for their own subsistence, although they 
are of a very diminutive size when thus cast off. 


them passage; after which, the lacerated part 
soon heals. 

In the instances which have now passed under 
our review, the progeny is, at first, in direct 
communication with its parent, and does not 
receive the special protection of membranous 
envelopes, containing a store of nourishment for 
its subsequent growth. But in all the more 
perfect structures, both of animals and vege- 
tables, the germ is provided with auxiliary 
coverings of this kind, the whole together com- 
posing what is called a seed^ or an ovum: the 
former term being usually applied to vegetable, 
and the latter to animal productions; and in 
both cases the oi^an which originally contained 
them is termed the ovary. 

The formation and evolution of vegetable 
seeds takes place, not indiscriminately at every 
point, as we have seen is the case with simple 
germs, but only in particular parts of the plant. 
The Filices^ or fern tribes, may be taken as 
examples of this mode of reproduction, the seeds 
being formed at the under surface of the leaves, 
apparently by a simple process of evolution; 
and when detached and scattered on the ground, 
being further developed into a plant similar to 
the parent. The Linnean class of Ciifptogamia 
includes all the plants coming under this de- 
scription. In Animals, likewise, it is only in 
the particular organs termed ovatiesj that ova 



are formed, and they are generally divided into 
compartments, the whole being enclosed in a 
membranous covering, bearing a great reeem- 
blance to the seed-capsules of plants. 

The propagation of living beings by means of 
•ova or seeds, is a process of a totally different 
class from their multiplication by mere slips or 
buds; and the products of the former retain 
less of the peculiar characters of the individual 
from which liiey spring, than those of the latt^. 
This is remarkably exemplified in the case of 
orchard trees, such as apples and pears ; for all 
the trees which derive their origin from shoots, or 
grafts from the same individual, partake of the 
same properties, and produce a fruit of the same 
flavour and qualities ; whereas trees of the same 
species, whieh grow from seed, have the charac* 
ters of distinct individuals, and losing all the 
peculiarities that may have distinguished the 
parent, revert to the original type of the species 
to which they belong. Thus from the seeds of 
the golden pippin, or nonpareil, arise trees bear- 
ing the common crab apple, which is the natural 
fruit of the species. By continued graftings^ 
after a long period, the vitality of the particular 
variety is gradually exhausted, and the grafts fio 
longer bear fruit. This has already happened 
with regard to the two varieties of apples just 
mentioned. For these curious facts, and the 
theory which explains them, we are indebted to 


the observation and sagacity of Mr. Andrew 

The plans hitherto noticed are suited only to 
the simplest of vegetable or animal beings ; but 
for the continnance of the higher races in both 
kingdoms of nature there is required a more 
complex procedure. The latent germ, contained 
in the seed or ovum, is never developed beyond 
a certain point, unless it be vivified by the action 
of a pectdiar fluid, which is the product of other 
organs. Thus thei^ are established two distinct 
classes of structures ; the office of the one being 
the formation of the seed or ovum, and that of 
the other the production of the vivifying fluid. 
The effect of this vivifying fluid upon the dor- 
mant germ is termed Fecundatuni; and the 
germ, when fecundated, receives the name of 

The modes in which the fecundation of the 
germ is accomplished are exceedingly various in 
different classes of organized beings. In all 
Pkaneroganuyus plants^ (so named in contra- 
distinction to those which are Cryptogamous)^ 
the whole of the double apparatus required for 
reproduction is contained in the flower. One 
set of organs contains the rudiment of the seed, 
enclosed in various envelopes, of which the as- 
semblage constitutes an ovary, and to which is 

* See his various papers in the Philosophical Transactions. 


appended a tube, (the pistil)^ terminated by a 
kind of spongiole, (the stigma). The fecunda^ 
ting organs are the stamensy which are columns, 
{or filament$)y placed generally near and parallel 
to the pistil, and terminated by a glandular 
organ, (the anther). This organ, when mature, 
contains, enclosed in a double envelope, a fine 
powder, (the pollen)^ consisting of very minute 
vesicles, filled with a viscous liquor, (thejbvtlla)^ 
in which are seen extremely small granules. 
Fecundation takes place by a portion of the 
pollen being received by the stigma, and con- 
veyed through the tubular pistil to the seed, 
which it impregnates by imparting to it the fluid 
it contains. 

By far the greater number of plants com- 
posing the vegets^ble kingdom have these two 
sets of organs contained in the same flower ; or 
at least in flowers belonging to the same indivi- 
dual plant. In the animal kingdom this ar- 
rangement is also adopted, but only in a com- 
paratively small number of tribes. In these the 
ova, in their passage from the ovary, along a 
canal termed the oviduct^ are fecundated by 
receiving a secretion from another set of organs 
in the same system-, which is conveyed by a 
duct, opening into the oviduct in some part of its 
course. In a limited number of plants, com- 
posing the class Dioecia, the individuals of the 
same species are distinguished by their bearing 


flowers which contain only one of the kinds of 
reproductive apparatus: so that the stamens and 
the pistils are situated on separate plants : and 
the impregnation of the ovaries in the latter, can 
be effected only by the transference of the pollen 
from the former. A similar separation of offices 
is established among all the higher classes of the 
animal kingdom. In most Fishes, and in all 
Batrachian reptiles, the ova are impregnated 
after their expulsion from the body : in all other 
cases their impregnation is internal, and their 
subsequent developement takes place in one or 
other of the four following ways. 

1 . The ovum, when defended by a firm enve- 
lope, which contains a store of nutriment, is 
termed an eggy and is deposited in situations 
most favourable for the developement of the 
embryo ; and also for its future support when it 
emei^es from the egg. Birds, as is well known, 
produce eggs which are encased in a calcareous 
shell, and hatch them by the warmth they com- 
municate by sitting on them with unwearied 
constancy. All animals which thus lay eggs 
are termed oviparous. 

2. There are a few tribes, such as the Viper 
and the Salamander^ whose ^gs are never laid, 
but are hatched in the interior of the parent ; so 
that they bring forth living offspring, although 
originally contained in eggs. Such animals are 
said to be Ovo-viviparous. There are other 


tribes, again, which, according to circumBtances, 
are either oviparous, or ovo-viviparous : this is 
the case with the Shark. 

3. Viviparous animals are those in which no 
^S& properly so called, is formed ; but the 
ovum, after proceeding through the oviduct, 
sends out vessels, which form an attachment to 
the interior of a cavity in the body of the parent, 
whence it draws nourishment, and therefore has 
attained a considerable size at the time of its 

4. Marsupial animals are those, which, like 
the Kangurooy and the Opossum^ are provided 
with abdominal pouches, into which the young, 
bom at a very early stage of developement, ore 
received, and nourished with milk, secreted firpm 
glands contained within these pouches. As the 
young, both in this and in the last case, are nou- 
rished with milk prepared by similar glands, or 
MamnuBt the whole class of viviparous and mar- 
supial animals has received, from this chuac- 
teristic circumstance the name of Mammalia. 


Chapter IL 


Alt^ouoh the study of organic structures in 
their finished state must tend to inspire the 
most sublime conceptions of the Great Creator 
of this vast series of beings, extending from the 
obscurest plant to the towering tenant of the 
forest, and from the lowest animalcule to the 
stately elephant and gigantic whale, there yet 
eicists another department of the science of 
Nature, removed, indeed, from the gaze of ordi- 
naiy obserters, but presenting to the philosophic 
inquirer subjects not less replete with interest, 
and not less calculated to exalt our ideas of the 
transcendent attributes of the Almighty. To a 
mind nurtured to reflection, these diyine attri- 
butes, whether of power, of wisdom, or of bene- 
ficence, are no where manifested with greater 
distinctness, or arrayed in greater glory, than in 
the formation of these various beings, and in 
the progressive architecture of their wondrous 

Our attention has already been directed, in a 
former part of these inquiries, to the successive 


changes which constitute the metamorphoses of 
winged insects,* and of Batrachian reptiles, 
phenomena which are too striking to have 
escaped the notice of the earliest naturalists: 
but the patient investigations of modem inquirers 
have led to discoveries still more curious, and 
have shown, that all vertebrated animals, even 
those belonging to the higher classes, such as 
birds, and mammalia, not excepting man him- 
self, undergo, in the early stages of their deve- 
lopement, a series of changes fully as great and 
as remarkable as tliose which constitute the 
transformations of inferior animals. They have 
also rendered it extremely probable that the 
organs of the system, instead of existing simul- 
taneously in the germ, arise in regulated succes- 
sion, and are the results not of the mere expan- 
sion of pre-existing rudiments, but of a real 
formation by the union of certain elements; 
which elements are themselves successively 
formed by the gradual coalescence or Juxta- 
position of their constituent materials. On con- 
templating the infinitely lengthened chain of 
means and ends, and of causes and effects, 
which, during the construction and assemblage 
of the numerous parts composing the animal 

* The researches of Nordmaan, on difFereDt species of Lertusa, 
have brought to light the most singular succession 6f forms 
during the progress of developement of the same individual 


machine, are in constant operation, adapting 
them to their various . purposes, and combining 
them into one efficient and harmonious system, 
it is impossible not to be deeply impressed with 
the extent and the profoundness of the views of 
Omniscient Providence, which far exceed the 
utmost boundaries of our vision, and surpass 
even the powers of the human imagination.* 
- The clearest evidence of enlarged and provi- 
dent designs may be collected from observing the 
order in which the nascent organs are succes- 
sively brought forwards, and added to the grow- 
ing fabric : such order appearing, in all cases, to 
be that best calculated to secure the due per* 
formance of their appointed functions, and to 
promote the general objects of the system. The 
apparatus first perfected is that which is imme- 
diately necessary for the exercise of the vital 
actions, and which is therefore required for the 
completion of all the other structures ; but pro- 

* '* Si I'on applique/' says Cuvier, when speaking of the ana- 
tomy of insects, " k chacune de ces esp^ces, par la pensee, ce 
qu*il seroit bien impossible qu'un homme entreprit de verifier en 
effet pour toutes, une organisation k-peu-pr^s 4gale en complica-* 
tion k celle qui a kt^ d^crite dans la chenille par Lyonet, et 
tout recemment dans le hanneton par M. Straus, et cependant 
plus ou moins difierente dans chaque insecte, TiQiagination 
commencera k concevpir quelque chose de cette ricbesse ef- 
frayante, et de ces millions de millions de parties, et de parties 
de parties, toujours correlatives, toujours en harmonie, qui con- 
stituent le grand ouvrage de la nature." (Histoire des Progr^s 
des Sciences Naturelles^ iv. 145.) 


vision is likewise made for the establishment of 
those parts which are to give mechanical sup- 
port to each organic system in proportion as it 
is formed; white the foundations are also pre- 
paring for endowments of a higher kind, by the 
early developement of the organs of the external 
senses, the functions of which so essentially 
minister to the futaie expansion of the intellec- 
tual faculties, embracing a wide range of per- 
ceptions and of active powers. Thus in the 
early, as well as in all the subsequent periods of 
life, the objects of nature vary as the respective 
necessities of the occasion change. At first, all 
the en^gies of vitality are directed to the raising 
of the fabric, and to the extension of those 
organs which are of greatest immediate utility ; 
but still having a prospective view to farther and 
more important ends. For the accomplishment 
of this primary object unremitting exertions are 
made, commensurate with the magnitude of the 
design, and giving rise to a quick succession of 
varied forms, both with regard to the shape of 
each individual organ, and to the general aspect 
of the whole assemblage. 

In the phenomena of their early evolution. 
Plants and Animals present a striking contrast, 
corresponding to essential difier^iees in the 
respective destinations of these two orders of 
beings. The primary object of vegetable struc- 
tures appears to be the establishment of the 


functions of nutrition ; and we accordingly find 
that whenever the seed begins to germinate, the 
first indication of developement is the appear^ 
aiice of the part called the plumula^ which is a 
collection of feathery fibres, bursting from the 
enveloping cs^sule of the germ, and which, 
whatever may have been its original position, 
proceeds immediately to extend itself vertically 
upwards ; while, at the same time, slender fila- 
ments, or radicles^ shoot out below to form the 
roots* Thus early are means provided for the 
absorption and the aeration of the nutrient 
matter, which is to constitute the materials for 
the subsequent growth of the plant, and for the 
support and protection of the organs by which 
these processes are to be carried on. But animal 
vitality, being designed to minister to a higher 
order of endowments, is placed in subordination 
to a class of functions, of which there exists no 
trace in vegetables, namely, those of the nervous 
system. By intently watching the earliest dawn 
of organic formation, in the transparent gelatinous 
molecule, for example, which, with its three in-* 
vesting pellicles, constitutes the embryo of a 
bird, (for the eggs of this class of animals best 
admit of our following this interesting series of 
changes,) the first opaque object discoverable by 
the eye is a small dark line, called the primitive 
trace, formed on the surface of the outermost 
pellicle. Two ridges then arise, one on each 


side of this dark line^; and by the union of 
their edges, they soon form a canal, containing a 
deposit of semi-fluid matter, which, on acquiring 
greater consistence and opacity, discloses two 
slender and delicate threads, placed side by 
side, and parallel to one another, but separated 
by a certain space. These are the rudiments of 
the spinal cord, or the central organ of nervous 
power, on the endowme;its of which the whole 
character of the being to be formed depends. 
We may next discern a number of parallel equi- 
distant dots, arranged in two rows, one on the 
Outer side of each of the filaments already no- 
ticed : these are the rudiments of the vertebrse, 
parts which will afterwards be wanted for giving 
protection to the spinal marrow, and which soon 
form, for this purpose, a series of rings embracing 
that organ. t 

The appearance of the elementary filaments 
of the spinal cord is soon followed by the deve- 
k)pement of its upper or anterior extremity, from 
which there arise three vesicles, each forming 
white tubercles ; these are the foundations of the 
future brain. The tubercles are first arranged 

* The plicm primitiv<B of Pander; the lamincR darsales of 
Baer. See a paper on embryology by Dr. AUea Thomson, in 
the Edin. New Phil. Journal for 1830 and 1831. 

t These rings have, by speculative physiologists, been sup- 
posed to be analogous to those which form the skeleton of the 


in pairs and in a longitudinal series, like those 
"we haye seen constituting the permanent form 
of the brain in the inferior fishes : but, in birds, 
they are soon folded together into a rounded 
mass ; while, in the mean time, the two filaments 
of the spinal cord have approached each other, 
and united into a single column, the form which 
they ever after retain. Even at this early period 
the rudiments of the organs of the higher senses, 
(first of the eye, and next of the labyrinth of the 
ear,) make their appearance : but, on the other 
hand, those of the legs and wings do not show 
themselves until the brain has acquired greater 
solidity and developement. The nerves which 
are to connect these organs of sensation and of 
motion with the spinal cord and brain are formed 
afterwards, and are successively united to the 
nervous centres. 

Although the plan of the future edifice has 
thus been sketched, and its foundations laid in 
the homogeneous jelly by the simpler efforts of 
the vital powers, the elevation of the vast super- 
structure demands the aid of other machinery, 
fitted to collect and distribute the requisite 
materials. Here, then, we might, perhaps, 
expect to meet with a repetition of those vege- 
tative processes, having similar objects in view, 
and the adoption of analogous means for their 
accomplishment ; but so widely different in cha- 
racter is the whole organic economy of these two 


orders of beings, that we perceive no resem- 
blance in the mechanism employed for their 
formation. For the purposes of animal life the 
nutrient jnices must be brought into active circn* 
lation by means of vessels extensively pervading 
the system. Nature, then, hastens to prepare 
this important hydraulic apparatus, without 
which the work of construction could not pro- 
ceed. What may be the movements of the 
transparent nutrient juices at the very earliest 
period must, of course, remain unknown to us, 
since we can only follow them by the eye after 
the nutritive substance they contain has become 
consolidated in the fonn of opaque globules. 
These globules are at first seen to meander 
through the mass, unconfined by investing ves- 
sels ; presently, however, a circular vessel is dis- 
covered, formed by the foldings of the membrane 
of the embryo, along which the fluids undulate 
backwards and forwards, without any con- 
stancy.* A delicate net-work of vessels is next 
formed in various parts of the area of the ciTcle> 
which are seen successively to join by the for- 
mation of communicating branches, and ulti- 
mately to compose larger trunks, so as to 
establish a more general system of vascular orga- 
nization. But increased power for carrying on 

* These phenomena are similar to those which were noticed 
as presented by the iarvee of some insects and other inferior 


this extended cireolation will soon be wanted; 
and for this pu]^x>se there must be provided a 
central organ of propulsion, or heart, the con^ 
struction of which is now commenced, at a 
central point, by the folding inwards of a lamina 
of the middle membrane, forming first a simple 
groove, but, after a time, converted, by the 
union of its outer edges, into a kind of sac, 
which is soon extended into a longitudinal tube.* 
The next object is to bring this tube, or nidi- 
mental heart, into communication with the 
neighbouring vascular trunks, and this is efiected 
by their gradual elongation, till their cavities 
meet, and are joined ; one set of trunks (the 
future veins,) first uniting with the anterior end 
of the tube ; and then another set (the future 
arteries,) joining its other end. The addition of 
this central tube to the vessels previously formed 
completes the continuity ^of their course : so that 
the uniform circulation of the blood is esta- 
blished in the direction in which it is ever after 
to. flow; and we may now recognise this central 
organ as the heart, which, under the name of the 
punctum salienSf testifies by its quick and regular 
pulsations that it has already begun to exercise its 
appropriate function. It is long, however, before 
it acquires the form which it is permanently to 

* The discovery of this fact is due to Pander. See also the 
works of Rolando, Wolff, Prevost and Dumas, and Serres. 


retain ; for from being at first a mere lengthened 
tube, presenting three dilatations, which are the 
cavities of the future auricle, ventricle, and bulb 
of the aorta, it assumes in process of time a 
rounded shape, by the folding of its parts, the 
whole of which are coiled, as it were, into a 
knot, by which means the different cavities 
acquire relative situations more nearly corre- 
sponding to their positions in the developed and 
finished oi^an. 

The blood-vessels, in like manner, undergo a 
series of changes quite as considerable as those 
of the heart, and totally altering their arrange- 
ment and distribution. Serres maintains that 
the primitive condition of all the organs, ev^i 
those which are generally considered as single, 
is that of being double, or being formed in pairs-; 
one on the right, and another exactly similar to 
it on the left of the middle, or mesial plane, as 
if each were the reflected image of the other.* 

* A remarkable exemplification of this tendency to symmetric 
duplication of organs occurs in a very extraordinary parasitic 
animal, which usually attaches itself to the gills of the Cyprintu 
brama, and which has been lately examined by Nordmann, and 
named by him the Diplozoon paradoxum^ from its having the 
semblance of two distinct animals of a lengthened shape, each 
bent at an obtuse angle, and joined together in the form of the 
letter X. The right and left halves of this cross are perfectly 
similar in their organization, having each a complete and inde- 
pendent system of vital organs, excepting that thestwo alimentary 
canals join at the centre of the cross to form a single cavity, or 
stomach. (Annales des Sciences Naturelles, nxx. 373.) 


Such is obviously the permanent condition of all 
the oi^ans of sensation, and also of the appa- 
ratus for locomotion : and it has just been shown 
that those portions of the nervous system which 
are situated in the mesial plane, such as the 
spinal cord and the brain, consisted originally of 
two separate sets of parts, which are brought 
together and conjoined into single organs. In 
like manner we have seen that the constituent 
laminae of the heart are at first double, and 
afterwards form by their union a single cavity. 
The operation of the same law has been traced 
in the formation of those vascular trunks, situated 
in the mesial plane, which are usually observed 
to be single, such as the aorta and the vena 
cava: for each were originally formed by the 
coalescence of double vascular trunks running 
paraUel to each other, and at first separated by 
a considerable interval ; then approaching each 
other, adhering together, and quickly converted, 
by the obliteration of the parts which are in 
contact, into single tubes, throughout a consider- 
able portion of their length.* 

Nature, ever vigilant in her anticipations of 

• These facta were first observed by Serres (Annales des Sc. 
Nat. xxi. 8.), and their accuracy has been confirmed by the ob- 
serrations of Dr. Allen Thomson. In Reptiles this union of the 
two constituent trunks of the aorta is effected only at the pos- 
terior part, while the anterior portion remains permanently 
double. (See Fig, 357, vol. ii. p. 274.) 



the wants of the system, has accumulated round 
the embryo ample stores of nutritive matter, suf- 
ficient for maintaining the life of the chick, and 
for the building of its frame, while it continues 
in the e^, and is consequently unable to obtain 
supplies from without : yet, with the same fore- 
sight of future circumstances, she delays not, 
longer than is necessary for the complete esta- 
blishment of the circulation, to construct the 
apparatus for digestion, on which the animal is 
to rely for the means of support in after life. 
The alimebtary canal, of which no trace exists 
at an earlier period, is constructed by the for- 
mation of two laminae, arising from folds of the 
innermost of the pellicles which invest the 
embiyo ; that is, on the surface opposite to the 
one which has produced the spinal marrow* 
These laminae, which are originally separate, 
and apart from one another, are brought tc^e- 
ther, and by the junction or soldering of their 
opposite edges, formed into a tube,* which, from 
being at first uniform in diameter, afterwards 
expands into several dilated portions, cone- 
spending with the cavities of the stomach, crop, 
gizzard, &c. into which they are to be converted, 
when the time shall come for their active em- 
ployment. These new organs are, however, even 
in this their rudimental state, trained to the per- 

* Wolff ig the author of this discovery. 


formance of their proper ofBcea^ receiving into 
their cavities, through a tube temporarily pro- 
vided for that t)urpo8e, the fluid of the yelki and 
preparing nourishment from it. 

Ih the m^an tiihe, early provision is made fot 
the aeration of the fluids by an extensive but 
temporary system of vessels, spread over the 
membrane of the egg, and receiving the influ- 
ence of atmospheric oxygen through the sub- 
stance of the shell, which is sufficiently porous 
to transmit it ; and these vessels, being brought 
into communication with the circulatory system 
of the chick, convey to its blood this vivifying 
agent. As the lungs cannot cOme into use till 
after the bird is emancipated from its prison^ 
and as it was sufficient that they be in readiness 
at that epoch, these organs are among the last 
which Bxe constructed : and as the mechanism 
of respiration in this class of animals does not 
require the play of the diaphragm, this muscular 
partition, though begun, is not completed, and 
there is no separation between the cavities of the 
thorax and the abdomen. 

The succession of organic metamorphoses is 
equally remarkable in the formation of the 
diversified apparatus for aeration, which is re- 
quired to be greatly modified, at different periods, 
in order to adapt it to different elements : of this 
we have already seen examples in those insect^ 
which, after being aquatic in their larva state, 


Emerge from the water when they have acquired 
'wings ; and also in the steps of transition from 
the tadpole to the frog. But similar, though less 
conspicuous changes occur in the higher verte* 
brated animals, during the early periods of their 
formation, corresponding to the differences in 
the modes of aeration employed at different 
stages of developement. In the primeval con* 
ditions this function is always analogous to that 
of aquatic animals, and requires for its perform- 
ance only the simpler form of heart already 
described, consisting of a single set of cavities : 
but the system being ultimately designed to 
exercise atmospheric respiration, requires to be 
gradually adapted to this altered condition ; and 
the heart of the Bird and the Quadruped must 
be separated into two compartments, correspond- 
ing to the double function it will have to perform. 
For this purpose a partition wall must be built 
in its cavity ; and this wall is accordingly begun 
around the interior circumference of the ven- 
tricle, and is gradually carried on towards the 
centre, there being, for a time, an aperture of 
communication between the right and left cavi* 
ties; but this aperture is soon closed, and the 
ventricle is now effectually divided into two. 
Next the auricle, which at first was single, 
becomes double ; not, however, by the growth of 
a partition, but by the folding in of its sides, 
along a middle line, as if it were encompassed 


by a cord, which was gradually tightened. lo 
the mean while the partition, which had divided 
the ventricle, extends itself into the trunk of the 
main artery, which it divides into two channels ; 
and these afterwards become two separate ves- 
sels; that which issues from the left ventricle 
being the aorta ; and the other, which proceeds 
from the right ventricle, being the pulmonary 
artery ; and each being now prepared to exercise 
its appropriate function in the double circulation 
which is soon to be established.^ 

A mode of subdivision of blood vessels, very 
similar to that just described, takes place in 
those which are sent to the first set of organs 
provided for aeration, and which resemble 
branchiae. These changes may be very dis- 
tinctly followed in the Batrackia ;t for we see, 
in those animals, the trunk of the aorta under- 
going successive subdivisions, by branches sent 
off from it and forming loops, which extend in 
length and are again subdivided, in a manner not 
unlike the unravelling of the strands of a rope ; 
each subdivision, however, being preceded by 
the formation of a double partition in the cavity 
of the tube ; so that at length the whole forms 
an extensive ramified system of branchial arte- 

* The principal authorities for the facts here stated are Baer 
and Rolando. See the paper of Dr. Thomson already quoted. 

t See the investigations of Rnsconi, and of Baer, on this 


ries and yeins. Still all these are merely tem- 
porary structures ; for when the period of change 
approaches, and the hranchise are to be super* 
seded in their office, every ressel, one after 
another, becomes obliterated, and there remain 
only the two original aortas, which unite into a 
single trunk lower down, and from which pro- 
ceed the pulmonary arteries, conveying either 
the whole, or a portion of the blood, to 
the newly developed respimtory organs, the 

By a similar process of continued bifurcation, 
or the detachment of branches in the form of 
loops, new vessels are developed in other parts 
6f the body, as has been particularly observed 
in the finny tail, and the external gills of the 
frog, and the newt, parts which easily admit of 
microscopical examination.* 

Progress is in the mean while making in the 
building of the skeleton, the forms of the prin- 
cipal bones being modelled in a gelatinous sub- 
stance, which is converted into cartilage, begin- 
ning at the surface, and gradually advancing 
towards the centre of each portion or element of 
the future bone; and thus a temporary solid 
and elastic scaffolding is raised, suited to the 

* Such is the result of the concurring observations of Spallan- 
ni, Fontana, and Dollinger. 



yielding texture of the nascent organs :. lastlyt 
the whole fabric is surrounded by an outer waU> 
the building of which is begun from the dorsal 
region, and conducted round the sides of. the 
body, till the two portions, come to meet ia the 
middle abdominal line, where they are finaUy 
united into one. general and continuous integu* 
ment. The eyes, which were hitherto unprot 
tected, receive special means of defence, by the 
addition of eyelids, which are formed by a 
farther extension and folding of these . into^ 
guments; and the greater part of the surface 
of the body gives rise to a growth of teipporary 
down, which, as we have seen, is provided as a 
covering to the bird at the time it is ready to 
quit the shell. But this hard shell, which had 
hitherto afforded it protection, is now opposed to 
its emancipation; and the chick, in order to 
obtain its freedom, must, by main force, break 
through the walls, of its prison ; its beak is, how- 
ever, as yet too tender to apply the force requi- 
site for that purpose. Here, again, we find 
Nature expressly interposing her assistance ; for 
she has caused a pointed horny projection . to 
gi'ow at the end of the beak, for the special 
object of giving the chick the power of batter- 
ing its shell, and making a practicable breach^ 
through which it shall be able to creep out, and 
begin its new career of life. That this horn is 


im>Yided only for this temporary use appears 
from the circumstance of its falling off spon- 
taneously in the course of three or four days 
after it has been so employed. 

But though the bird has now gained ite 
liberty, it is still unable to provide for its own 
maintenance, and requires to be fed by its pa- 
rent till it can use its wings^ and has learned 
the art of obtaining food. The p^eon is fur- 
nished by nature with a secretion from the crop, 
with which it feeds its young. In the Mammalia 
the same object is provided for still more ex- 
presdy, by means of glands, whose office it is to 
prepare milk^ a fluid which, from its chemical 
qualities, is admirably adapted to the powers of 
the digestive oi^ns, when they first exercise 
their functions. The Cetacea have also mam- 
mary glands ; but as the structure of the mouth 
and throat of the young in that class does not 
appear adapted to the act of sucking, there has 
always been great difficulty in understanding 
how they obtain the nourishment so provided. 
A recent discovery of Geoftroy St. Hilaire ap- 
pears to have resolved the mystery with respect 
to the Delphinus glohiceps, for he found that the 
mammary glands of that animal contain each 
a large reservoir, in which milk is accumulated, 
and which the dolphin is capable, by the action 
of the surrounding muscles, of emptying at once 


into the mouth of its young, without requiring 
from the latter any effort of suction.* 

The rapid sketch which I have attempted to 
draw of the more remarkable steps of the early 
stages of organic developement in the higher 
animals, taken in conjunction with the facts al- 
ready adverted to in various parts of this Trear 
tise, and particularly those relating to ossifica* 
tion, dentition, the formation of hair, of the quills 
of the porcupine, of the antlers of the stag, and 
of the feathers of birds, will suffice to show that 
they are regulated by laws which are definite, 
and preordained according to the most enlarged 
and profound views of the future circumstances 
and wants of the system. The double origin of all 
the parts of the frame, even those which appear 
as single organs, and the order of their forma* 
tion, which, in each system, commences with the 
parts most remote from the centre, and proceeds 
inwards, or towards the mesial plane, are among 
the most singular and unexpected results of this 
train of inquiries.f We cannot but be forcibly 

* The account of this discovery is contained in a memoir \vhich 
was read at the '* Insticut/' March 24, 1834. 

t The first of these two laws is termed by Serres, who has 
zealously prosecuted these investigations, '* la hi de symmStrie ;** 
and the second, *' la loi de conjugaison" He maintains that 
they are strictly applicable to all the parts of the body having a 
tubular form, such as the trachea, the Eustachian tube, the canals 
and perfordtions of bones, &c. See the preliminary discourse to 


Struck with the numerous forms of transition 
through which every organ has to pass before 
arriving at its ultimate and comparatively per- 
manent condition : we cannot but wonder at the 
vast apparatus which is provided and put in 
action for effecting all these changes ; nor can 
we oveilook the instances of express contrivance 
in the formation of so many temporary struc- 
tures, which are set up, like the scaffold of an 
edifice, in order to afford the means of trans- 
porting the materials of the building in proper* 
tion as they are wanted ; nor refuse to recognise 
the evidence of provident design in the regular 
order in which the work proceeds, every organ 
growing at its appointed time, by the addition 
of fresh particles brought to it by the arteries, 
while others are carried away by the absorbents, 
and gradually acquiring the form which is to 
qualify it for the performance of its proper 
office in this vast and complicated system. 

his ** Anatomie comparee du cerreau/' p. 25 ; and also his se- 
veral memoirs in the '* Annales des Sciences Naturelles/' vols. xi. 
xii. xvi. and xxi. 

An excellent summary of the principal facts relating to the 
developement of the embryo is given by Mr. Herbert Mayo, in 
the third edition of his *' Outlines of Human Physiology." 


Chapter III. 


To follow miirately the various steps by which 
Nature conducts the individual to its state of 
maturity, would engage us in details incom- 
patible with the limits of the present work. 
I shall only remark, in general, that during the 
period when the body is intended to increase in 
size, the powers of assimilation are exerted to 
prepare a greater abundance of nourishment, so 
that the average supply of materials rather ex- 
ceeds the consumption : but when the fabric has 
attained its prescribed dimensions, the total 
quantities furnished and expended being nearly 
balanced, the vital powers are no longer exerted 
in extending the fabric, but are employed in 
consolidating and perfecting it, and in qualifying 
the organs for the continued exercise of their 
respective functions, during a long succession of 

Yet, while every function is thus maintained 
in a state of healthy equilibrium, certain changes 
are in progress which, at the appointed season. 


will inevitably bring on the decline, and ulti- 
mate destruction of the system.* The process 
of consolidation, begun, from the earliest period 
of developement, is still advancing, and is pro- 
ducing in the fluids greater thickness, and a 
reduction of their total quantity; and in the 
solids, a diminution in the proportion of gelatin, 
and the conversion of this element into albumen. 
Hence all the textures acquire increasing so- 
lidity, the cellular substance becomes firmer and 
more condensed, and the solid structures more 
rigid and inelastic : hence the tendons and liga- 
mentous fibres growing less flexible, the joints 
lose their suppleness, and the contractile power 

* It would appear from the researches of De Candolle, that 
the Yegetable system is not, like the animaU subject to Uie 
destructive operation of internal causes; for the agents which 
destroy v^etable life are always extraneous to its economy. 
Each individual tree is composed of an accumulation of the shoots 
of every successive year since the commencement of its growth ; 
and although, from the continued deposition of lignia, and the 
consequent obliteration of many of its cells and vessels, the vi- 
tality of the interior wood may be destroyed, and it then becomes 
liable to decay by the action of foreign agents, yet the exterior 
layers of the /t6er still vegetate with undiminished vigour ; and, 
unless injured by causes extraneous to its own system, the life of 
the tree will continue to be sustained for an indefinite period. 
If, on the other hand, we were to regard each separate shoot as an 
individual organic body, and every layer as constituting a dis- 
tinct generation of shoots, the older being covered and enclosed 
in succession by the younger, the great longevity of a tree would, 
on this hypothesis, indicate only the permanence of the species, 
not the indefinitely protracted duration of the individual plant. 


being al&o impaired, the muscles act more tardily 
as well as more feebly, and the limbs no longer 
retain the elastic spring of youth. The bones 
themselves grow harder and more brittle; and 
the cartilages, the tendons, the serous mem- 
branes, and the coats of the blood-vessels, ac- 
quire incrustations of ossific matter, which inter- 
fere with their uses. Thus are all the progres- 
sive modifications of structure tending, slowly 
but inevitably, to disqualify the organs for the 
due performance of their functions. 

Among the most important of the internal 
changes consequent on the progress of age are 
those which take place in the vascular system. 
A large proportion of the numerous arteries, 
which were in full activity during the building 
of the fabric, being now no longer wanted, are 
thrown, as it were, out of employment ; they, in 
consequence, contract, and becoming impervious, 
gradually disappear. The parts of the body, no 
longer yielding to the power applied to extend 
them, oppose a gradually increasing resistance 
to the propelling force of the heart ; while, at 
the same time, this force, in common with all the 
others, is slowly diminishing. Thus do the vital 
powers become less equal to the demands made 
upon them ; the waste of the body exceeds the 
supply, and a diminution of energy becomes 
apparent in every function. 

Such are the insensible gradations by which, 


while gliding down the stream of time, we lapse 
into old age, which insidiously steals on us 
before we are aware of its appitmch. But the 
same provident power which presided at our 
birth, which superintended the growth of all the 
organs, which infused animation into each s.^ 
they arose, and which has conducted the system 
unimpaired to its maturity, is still exerted in 
adjusting the conditions under which it is 
placed in its season of decline. New arrange- 
ments are made, new energies are called forth, 
and new resources are employed, to accommo- 
date it to its altered circumstances, to prop the 
shattered fabric, and retard the progre^ of its 
decay. In proportion as the supply of nutritive 
materials has become less abundant, a more 
strict economy is practised with regard to their 
disposal ; the substance of the body is hus- 
banded with greater care 'f the absorbent vessels 
are employed to remove' such parts as are no 
longer useful ; and when all these adjustments 
have been miade, the functions still go on for 
a considerable length of time without material 

The period prescribed for its duration being 
at length completed, and the ends of its exist- 
ence accomplished, the fabric can no longer 
be sustained, and preparation must be made 
for its inevitable fall. In order to form a cor- 
rect judgment of the real intentions of nature. 


with regard to this last stage of life, its pheno* 
mena must be observed in cases where the sys- 
tem has been wholly entrusted to the operation 
of her laws. When death is the simple conse- 
quence of age, we find that the extinction of the 
powers of life observes an order the reverse of 
that which was followed in their eyolution. The 
sensorial functions, which were the last perfected, 
are the first which decay ; and their decline is 
found to commence with those mental faculties 
more immediately dependent on the physical 
conditions of the sensorium, and more especially 
with the memory, which is often much impaired, 
while the judgment remains in full vigour. The 
next faculties which usually suffer, from the 
dfects of age are the external senses, and the 
failure of sight and of hearing still farther con- 
tributes to the decline of the intellectual powers, 
by withdrawing the occasions for their exercise. 
The actual demolition of the fabric commences 
whenever there is a considerable failure in the 
functions of assimilation : but the more imme- 
diate cause of the rapid extinction of life is 
usually the impediment which the loss of the 
sensorial power, necessary for maintaining the 
movements of the chest, creates to respiration. 
The heart, whose pulsations gave the first indi- 
cations of life in the embryo, generally retains 
its vitality longer than any other organ ; but its 
powers being dependent on the constant oxida- 


tion of the blood in the lungs, cannot survive 
the interruption of this function; and on the 
heart ceasing to throb, death may then be consi- 
dered as complete in every part of the system. 

It is an important consideration, with refer- 
ence to final causes, that generally long before 
the commencement of this 

" Last scene of all, 
That ends this strange eventful history,*^ 

the power of feeling has wholly ceased, and the 
physical struggle is carried on by the vital 
powers alone, in the absence of all consciousness 
of the sentient being, whose death may be said 
to precede, for some time, that of the body. In 
this, as well as in the gradual decline of the sen- 
sorial fiEiculties, and the consequent diminution 
both of mental and of physical sensibility in 
advanced age, we cannot fail to recognise the 
wise ordinances c^ a superintending and bene- 
ficent providence, kindly smoothing the path 
along which we descend the vale of life, spread- 
ing a narcotic mantle over the bed of death, and 
giving to the last moments of departing sensa- 
tion the tranquillity of approaching sleep. 


Chapter IV. 


The inquiries on Animal and Vegetable Physi- 
ology in which we have been engaged, lead to 
the general conclusion that unity of design and 
identity of operation pervade the whole of nature ; 
and they cleai'ly point to one Great and only 
Cause of all things, arrayed in the attributes of 
infinite power, wisdom, and benevolence, whose 
mighty works extend throughout the boundless 
regions of space, and whose comprehensive plans 
elnbrace eternity. 

In examining the manifold structures and 
diversified phenomena of living beings, we can- 
not but perceive that they are extensively, and 
perhaps universally connected by certain laws of 
Analogy ; a principle, the recognition of which 
has given us enlarged views of a multitude 
of important facts, which would otherwise have 
remained isolated and unintelligible. Hence 
naturalists, in arranging the objects of their 
-study, according to their similarities and ana- 
logies, into classes, orders and genera, have but 

VOL. n. s s 


followed the footsteps of Nature harsdf, who in 
all her operations combines the apparently op- 
posite principles of general res^nblance, and of 
specific yariety; so that the races which she 
has united in the same group, though possessed 
of features individually different, may easily be 
recognised by their family likeness, as the off- 
spring of a common parent. 

** Facies non omnibus ana ; 
Nee divena tamen ; qualem decet esse soronim.** 

We hecve seen that in each of the two great 
divisions, or kingdoms of organic nature, the 
same general objects are aimed at, and the same 
general plans are devised for their accomplish* 
meAt ; and also that m the executikm of those 
plans siniilar means and agencies are employed. 
In each division there prevails a remarkable 
uniformity in the composition add properties of 
their elementary textures, in the nature of their 
vital powers, in th^ arrangement of their organs, 
and in the laws of their prodacticm and develops- 
ment. The same principle of analogy Tf»f be 
traced, amidst endless modifi^^ions of <ktail^ in 
all the subordinate glt^ups into which each 
kingdom admits of being subdivided; both in 
respect to the orgtoizatitm and functiofcis of 
the objects comprehended in each assemUage, 
whether we examine the wonderg . of their me*- 
chanical fabric, or study the series of processes b^ 
which nutrition, sensation, voluntary motion, and 


reproduction are effected. To specify all the 
examples which might be adduced in confinna* 
tioR of tins obyibus truth is here unnecessary; 
im it ironld be <mly to repeat the numerous facts 
fdready noticed in erery chapter of this treatise^ 
relative to each natural group of living beings : 
and it was» indeed, chiefly by the aid of such 
analogies^ that we were enabled to connect and 
generalize those facts« We have seen that, in 
constructing each of the divisions so established^ 
Nature appears to have kept in view a certain 
definite lype, or ideal standard, to which, amidst 
nnumemble modifications, rendered necessary 
by the var3ing circumstances and different des* 
tinations of each species, she always shows a 
decided tend^icy to conform. It would almost 
seem as if^ in laying the foundations of each m« 
ganized fabric, she had ccnnmenced by taking 
an exact copy of this primitive model ; and^ in 
building the superstructur^e, had allowed herself 
to depart fiom the originid plan only for the pur* 
pose of accommodation to certain specific an<l 
nltermr ob^ects^ conformably with the destina- 
tiou of that particular race of created beings; 
Such, indeed, is the hypothetical principle, 
which, under the title of unity ^ compantioHf 
has been adopted, and zealously piusued in all 
its coDBequences, by many naturalists, of the 
Inghest eminence, on the continent. As the 
fistets on which this hypothesis is supported, and 
the views which it unfolds, are highly deserving 


of attaation, I shall here briefly state them ; hnt 
in so doing I shall beg to premise the caution 
that these views should for the j^esent be re- 
garded as hypothetical, and as by no means pos- 
sessing the certainty of jrfiilosophical gen^rali^ 

The hypothesis in question is countenanced, 
in the first place, by the supposed constancy 
with which, in all the animals belonging to the 
same natural group, we meet with the same con* 
stituent el^nents of structure, in each respective 
system of oi^ns, notwithstanding the utmost 
diversity which may exist in the forms of the 
organs, and in the uses to which they are ap- 
plied. This principle has been most strikingly 
exemplified in the osteok^ of vertebrated ani- 
mals; but its truth is also inferred from the 
examination of the mechanical fabric of Insects, 
Crustacea, and Arachnida; and it appears to 
extend also to the structures subservient to other 
functions, and particularly those of the nervous 
System. Thus Nature has provided for the 
locomotion of the serpent, not by the creation 
of new structures, foreign to the type of the 
vertebrata, but by employing the ribs in this 
new office ; and in giving wings to a lizard, 
she has extended these same bones to serve as 
supports to the superadded parts. In arming 
the elephant with tusks, she has merely caiised 
two of the teeth in the upper j&tr to be developed 


into these formidable weapons ; and in providing 
it with an instrument of prehension has only; 
resorted to a greater elongation of the snout. 

The law of Gradation, in conformity to which 
all the living, together with the extinct races, of 
organic nature, arrange themselves, more or less, 
into certain regular series, is one of the conse- 
quences which have been deduced from the 
hypothesis we are considering. Every fresh 
copy taken of the original type is supposed to 
receive some additional extension of its faculties 
and endowments by the graduated developement 
of elements, which existed in a latent form in 
the primeval germ, and which are evolved, in 
succession, as nature advances in her course. 
Thus we find that each new form which arises, 
in following the ascending scale of creation, 
retain^ a strong affinity to that which had 
preceded it, and also tends to impress its own 
features on those. which immediately succeed; 
and thus their specific difierences result merely 
from the different extent and direction given to 
these organic developements ; those of inferior 
races proceeding to a certain point only, and 
there stopping; while in beings of a higher 
rank they advance farther, and lead to all the 
observed diversities of conformation and endow- 

It is remarked, in further corroboration of 
these views, that the animals which occupy the 

030 UMmr OP DESMN . 

bluest stalioiis in each aeries p oMCOB , at die 
commencement of their enBteoce^ feims exUint* 
ing a maiked reaemblance to those presented in 
the pennanent condition of the lowest animals 
in the same series; and that, daring the pnn 
grass of their developement, thej assume, in 
soccession, the characters of each tribe, cocpe- 
Impending to their conseentiTe order in the 
ascending chain : so that the pecmliaritieB which 
distinguish the higher animal, on its staining 
its nltimate and permanent form, are those 
which it had receiTed in its last stage of ^nbcy* 
onic erolntiMi. Another consequence of this 
hypothesis is that we may expect oocaaionally 
to meet, in inferior animals, with rudimental or- 
gans, which fiom their imperfect developement 
may be of Utde or no use to the indiridval, b«t 
which become ayailable to somesoperiw speities, 
in which they are sufficiently perfected. The 
following are among the most remarkaUe hcto 
in illustration of these propomtiiMis. 

In the series of Articulated Animals, ctf^ which 
the Annelida omstitute the lowest, and winged 
Insects the highest terms, we find that the kunrae 
of the latter are often scarcdy distinguishable, 
either in outward form, or in internal t»ganiaa* 
tion, from Vermes of the lower orders 7 both 
being equally destitute of, or but imperfectly 
provided with external instruinents of locomo- 
tion; both haying a distinct yascular 


and mukif^ organs of digestion ; and . the 
central. filaments of the nervous system in both, 
being stadded with numerous pain, of equidis- 
tant gangUa. . In the worm all these features 
r^^nain as permanent characters of the order: in 
the insect they are subsequently modified and 
altered during its progressive .metamorphoses. 
The embryo of a crab resembles in appearance 
the permanent fwms of the MyHapoday and of 
the lower animals of its own class, but acquires, 
in the progress of its growth, iiew parts; while 
those already evolved become more and more 
eohcentiated, passmg, in their progress, through 
all. the ; forms of transition which characterise 
the intermediate tribes of Crustacea; till, the 
annual attains its last state, and then exhibits 
tiie most developed condition of that particular 

However different the conformations of the 
Fish, Jtfae Reptile, the Bird, and the Warm 
blooded Quadruped, may be at the period o£ their 
maturity, they are scarcely distinguishable from 
cnie another in their embryonic state ; and their 
early . develc^ement proceeds for some time in 

* This curious analogy is particularly observable in the sue- 
cctoive fotms assumed by the nervous system, whieh exhibits* a 
gradual passage from that of the Talitrus, to its ultiooiate greajt- 
est concentration in the Maia, (See Figures 439 and 441, p. 
543 and 545.) Milne Edwards has lately traced a similar pro- 
gressiion oP developement in the organs of locomotion of the 
Cniflftaoea. (Annate^ des Bciences Naturelles; xxx, 354.) 


the same maimer. They all poeseM at first 
the characters of aquatic animals; and the 
Frog even retains this fonn for a consideraUe 
period after it has left the e^. . The young 
tadpole is in truth a fidi, whether we regard 
the form and actions of its instruments of pnH 
gressive motion, the arrangement <tf its wgans 
of circulation and of respirati<m, or the condkioa 
of the central organs of its nervous system. We 
have seen hy what gradual and curious tranai'-' 
tions all these aquatic characters are changed 
for those of a terrestrial quadruped, furnished 
with limbs for moymg on the ground, and with 
lungs for breathing atmospheric air; and how 
the plan of circulation is altered from branchial 
to pulmonary, in proportion as the gills wither 
and the lungs are developed. If, while this 
change is going on, and while both sets of 
oigans are together executing the function of 
aeration, all further developement were pre* 
vented, we should have an amphibious animal, 
fitted for maintaining life both in air and in 
water. It is curious that this precise condition 
is the permanent state of the Siren and the 
ProteuSy animals which thus exemplify one of 
the forms of transition in the metamorphoses of 
the Frog. 

In the rudimental form of the feet of serpents, 
which are so imperfectly developed as to be 
concealed underneath the skin, and to be use* 


less as organs of progressive motion , we have an 
example of the first stage of that process, whichy 
when carried farther in the higher animals, 
gives rise to the limbs of quadrupeds, and which 
it would almost seem as if nature had instituted 
with a prospective view to these more improved 
eonstructions. Another, and a still more re- 
markable instance of the same kind occurs in 
the rudimental teeth of the young of the Whale, 
which are concealed within the lower jaw, and 
which are afterwards removed, to give place to 
the curious filtering apparatus, which occupies 
the roof of the mouth, and which nature has 
substituted for that of teeth, as if new objects, 
superseding those at first pursued, had arisen 
in the progress of developement. 

Birds, though destined to a veiy different 
sphere of action from either fishes or reptiles, are 
yet observed to pass, in the embryonic stage of 
their existence, through forms of transition, which 
successively resemble these inferior classes. 
The brain presents, in its earliest formation, a 
series of tubercles, placed longitudinally, like 
those of fishes, and only assuming its proper 
character at a later period. The respiratory 
Cleans are at first branchiae, placed, like those of 
fishes, in the neck, where there are also found 
branchial apertures similar to those of the lam- 
prey and the shark ; and the heart and great 
vessels are <;onstructed like those of the tadpole, 


i?itb reference to a branchial circolation. In 
^eir coDveFBion to the purposes of aerial resfH* 
ration, they undergo a series of changes pre^ 
cisely analogous to those of the tadpole. 

Manunalia, during the early periods of their 
developement, are subjected to. all the transform- 
ations which have been now described, eaok^ 
mencing with an organization correspcHiduig to 
that of the aquatic tribes, exhibiting not only 
branchiae, supported on branchial aiches, but 
also branchial apertures in the neck, and thoice 
passing quickly to the conditions of structure 
adapted to a terrestrial existence. The deve- 
lopement of various parts of the system, more 
especially of the brain, .the ear, die mouth, jcnd 
the extremities, is carried still farther than in 
birds. Nor is the human embryo exempt fix>m 
the same metamorphoses, possesong at one 
period branchiae and branchial apertures similar 
to those of the cartilaginous fishes,* a heart with 
a single set of cavities, and a brain consisting of 
a longitudinal series of tubercles; next losing 
its branchiae, and acquiring lungs, while the 
circulation is yet single, and thus imitating liie 
condition of the reptile ; then acquiring a double 
circulation, but an incomplete diaphragm, like 
birds ; afterwards, appearing like a quadruped, 

* These facts are ^ven on the authorities of Ratbke, Baer, 
Huschke, Breschet, &c. Aon. des Sc, Naturelles, xv. 266. See 
also the paper of Dr. A. Thomson, already quoted. 


Hatb a caudal prolongation of theBdcrum, and 
an intetmaxillary bone ; and lastly, changing its 
flftrnctme to one adapted to the erpct position^ 
accompanied by a great expansion of the cerebral 
hemispheres, which extend backwards so as 
completely to coyer the cerebellmn. Thus does 
the whole &bric arriye, by a gradual process of 
mutation, at an extent of elaboration and refine- 
ment, unattained by any other race of terrestrial 
beings, and which has been justly regarded as 
constituting the climax of organic deydc^* 

It must, I think, be admitted that the analo- 
gies, on which the hypotbesis in question is 
fimnded, are numerous and striking ; but great 
care should be taken not to caiTy it farther than 
the Just interpretation of the &cts themselves 

* A popnlat opinion baa long {xrevailed, ev^ among the 
well informed, that miB-9hapea or monstroxis .productions, or 
lusus natura, as they were termed, exhibit but the freaks of 
nature, who was believed, on these occasions, capriciously to 
abandon her usual course, and to amuse herself in the production 
of grotesque being9> without any ap^iid. object. But it ia now 
found that all defective formations of this kind are occasioned 
by the imperfect developement of some parts of the embryo, 
while the natural process is carried on in the rest of the system ; 
and thm it happens that a resemblance may often be.tfai^, in 
Iheae malformations, with the type or the permanent oonditioa- 
of some inferior animal. Hence all these apparent anomalies 
am, in reality, in perfect harmony with the established laws of 
organic developement, and afford, indeed, striking confirmations 
of the truth of the theory here explained. 


may warrant. It should be borne in mind 
that these facts are few, compared with the 
entire history of animal developement ; and that 
the resemblances which have been so ingeniously 
traced, are partial only, and fall very short of 
that universality, which alone constitutes the 
solid basis of a strictly philosophical theory. 
Whatever may be the apparent similarity be- 
tween one animal and another, during different 
periods of their respective developements, there 
still exist specific differences, establishing be- 
tween them an impassable barrier of separation, 
and effectually preventing any conversion of one 
species into another, however nearly the two 
may be mutually allied. The essential charac- 
ters of each species, amidst occasional varieties, 
remain ever constant and immutable. Although 
gradations, to a greater or less extent, may be 
traced among the races both of plants and 
animals, yet in no case is the series strictly 
continuous ; each step, however short, being in 
reality an abrupt transition from one type of 
conformation to another. In many instances the 
interval is considerable ; as for example in the 
passage from the invertebrate to the vertebrated 
classes; and indeed in every instance where 
great changes in the nature and arrangement ci 
the functions take place.* It is in vain to allege 

♦ See a paper on this subject, by Cuvier, in the Ann. des 
Sciences Naturelles, xx. 241. 


that the original continuity of the series is indi* 
cated by a few species presenting, in some res- 
pects, intermediate characters, such as the Orni- 
tharhyncuSy between birds and mammalia, and 
the CetaceUy between fishes and warm blooded 
quadrupeds : fOr these are but detached links of 
a broken chain, tending, indeed, to prove the 
unity of the designs of Nature, but showing also 
the specific character of each of her creative 
efforts* The pursuit of remote and often fanciful 
analogies has, by many of the continental physi- 
ologists, been carried to an unwarrantable and 
extravagant length : for the scope which is given 
to the ima^nation in these seductive specu- 
lations, by leading us far away from the path of 
philosophical induction, tend^ rather to obstruct 
than to advance the progress of real knowledge. 
By ccmfining our inquiries to more legitimate 
objects, we shall avoid the delusion into which 
one of the disciples of this transcendental school 
appears to have fallen, when he announces, with 
exultation, that the simple laws he has discovered 
have now explained the universe ;* nor shall we 
be disposed to lend a patient ear to the more 
presumptuous reveries of another system-builder, 
whO) by assuming that there exists in organized 
matter an inherent tendency to perfectibility, 

* " Uunivers est explique, et nous le voyons ; c*est un petit 
nombre de principes g^n^raux et f^conds qui nous en ont donn6 
la clef." Serres, Ann. des Sc. Nat. xi. 50. 


fancies that he can* supersede the operations of 
Dirine agency.* 

Very different wan the humble spirit of tke 
great Newton^ who» struck mdi the immensity 
of nature^ compared ottr knowledge of her cfte^ 
ratidnS) into which hcihad himself penetrated so 
deeply; to that of a child gathering pebbles on 
the sea shor^ Compared, indeed, with the 
magnitude of the uniTerse, how narrow is th^ 
field of our perceptions, and how far distant 
from ahy approximation to a knowledge of thm 
essencd of matter, of the source of its powers^ or 
even of the ultimate configurations of its pavtei 
How remote fmn all human cogni2aiice are the 
intimate properties of those impandeiraMe agMits^ 
Light, Hbat^. and Electricity, which pervade 
space, and exercise so potent a control over aH 
the bodies in natureJ Doubtless thera emat 

* Allusion is here made to the celebrated theory of Lamarck, 
ar exposed ifr his ** Philosopbie ZtK>log{qiie." H^ ceiloeiTeft 
that there waa origiDaUy no diatioctioii of tpceies, butthat eack 
race has, in the course of ages, been derived from some other, 
less perfect tdan itself, by a spontaneous effort at improvement ; 
tilnd he su]7poses that infbsorial animalcnles, spontaneonsly 
formed out of organio molecoks, g^ve bittfa, by sacoMrive trans- 
formations, to all other animals now ejJstiog on the globe. He 
believes that trib^, originally aquatic, acquired by their own 
efforts, prompted by their desire to walk, both feet and legs, 
fitting them for progression oil Khe grbttnd ; and that Ui^te 
members, by the long continued operation of the wish to fly, 
were transformed into wings, adapted to gratify that desire. If 
this be philosophy, it is such aa might have emanated from the 
college of Laputa. 


around us, on every side, influences of a still 
more subtle kind, which '' eye hath not seen, nor 
ear heard," neither can it enter into the heart or 
imagination of man to conceive. How scanty is 
our knowledge of the puind; how incompre* 
hensible is its connexion with the \^y ; how 
mysterious are its secret springs, and inmost 
workings ! What ineffable wonders would burst 
upon us, were we admitted to the perception 
of the spiritual world, now encompassed by 
clouds impervious to mortal vision ! 

The Great Author of our being, who, while he 
has been pleased to confer on us the gift of 
reason, has prescribed certain limits to its 
powers, permits us to acquire, by its exercise, a 
knowledge of some of the wondrous works of his 
'Creation, to interpret the characters of wisdom 
and of goodness with which they are impressed, 
and to join our voice to the general chorus 
which prockmns '^ His Might, Majesty, and Do- 
minion." From the same graeious hand we also 
derive that unquenchable thirst for knowledge, 
which this fleeting life muSt ever leave unsatis- 
fied; those endowments of the inoral sense, 
-with which the present constitution of the world 
so ill accords; and that innate decdre of perr 
feetion which our present frail condition is so 
inadequate to fulfil. But it is iiot given to 
man to penetrate into the counsels, or fathoin 
the designs of Omnipotence ; for in directing 


views into futurity, the feeble light of his reason 
is scattered and lost in die vast sbyEs. Although 
we plainly discern intention in every part of the 
creation^ the grand object of the whole is placed 
far above the scope of our comprehension. It is 
impossible^ however, to conceiTe that this enor* 
Inous expenditurie of pow», this vast accumula^ 
tion of contrivances and of machinery, and this 
profusion of existence resulting from them, can 
thus, from age to age, be prodigally lavished, 
without some ulterior end. Is Man, the favoured 
creature of nature's bounty, ** the paragon of 
animals," whose spirit holds communion with 
celestial powers, formed but to perish with the 
wreck of his bodily frame? Are generations 
after generations of his race doomed to follow in 
endless succession, rolling darkly down the 
stream of time, and leaving no track in its path- 
less ocean? Are the operations of Almighty 
power to end with the present scene? May we 
not discern, in the spiritual constitution of man 
the traces of higher powers, to which those he 
now possesses are but preparatory ; some embryo 
faculties which raise us above this earthly habi* 
tation ? Have we not in the imagination, a power 
but little in harmony with the fetters of our 
bodny organs ; and bringing within our view 
purer conditions of being, exempt from the illu- 
sions of our senses and the infirmities of our 
nature, our elevation to which will eventually 


prove that all these unsated desires of know- 
ledge, and all these ardent aspirations after 
moral good, were not implanted in us in vain ? 

HappUy there has been vouchsafed to us, 
from a higher source, a pure and heavenly light 
to guide our faltering steps, and animate our 
fainting spirit, in this dark and dreary search : 
revealing those truths which it imports us 
most of aU to know, giving to morality higher 
sanctions, elevating our hopes and our affections 
to nobler objects than belong to earth, and 
inspiring more exalted themes of thanksgiving 
and of praise. 


I JV D E X. 

Abdomen of bsects, i. 324. 
Aberration, chromatic, ii. 474. 
Aberration of parallax, ii. 459, 

Aberration, spherical, ii. 45S, 

Absorption,yegetable,ii. 19,22. 
Absorption, animal, ii. 12, 351. 
Absorption, lacteal, ii. 226. 
Absorption of shell, i. 239. 
Acalepha, i. 192; ii. 293. 
Acarus, i. 297. 
Achatina zebra, i. 242. 
Achromatic power, ii. 475. 
Acephiia, i. 217 ; ii. 1 16, 300. 
Acetabulum, i. 405. 
Acid secretions, ii. '46. 
Acrida,.]i. 214. 
'Acridium, i. 333. 
Acoustic principles, ii. 414. 
Actinia, i. 182, 197; ii. 99, 

383, 477, 586, 592. 
Adipose substance, i. 123. 
'Adductor muscle, i. 218. 
Aeration of sap,'Ji. 29. 
Aeration, anim^l/ii« 34, 611. 
^schna, i. 351. 
Affinities, organjc, ii. 7. 
Agastric mednsee, ii. 92. 
Age of trees; i.*^5. 
Age, effects df, ii.'620. 
•Agouti^ i. 498. 
Agrion, ii. 240. 

Air-bladder, i. 429. 
Air cells of plants, i. 76. 
Air cells of birds, ii. 329. 
Air, rarefaction of, in birds, i. 

Air tubes in plants, i. 73. 
Albumen, i. 105. 
Alburnum, i. 85.; ii. 41. 
Algse, ii. 19. 

Alimentary canal, ii. 107. 
Alimentary canal, formation of, 

ii. 610. 
Alitrunk, i. 345. 
Alligator, i. 458, 460 ; ii. 409. 
Amble, i. 495. 
Ambulacra, i. 201. 
Amid, i. '77 ; ii. 50. 
Amphibia, i: 436, 487. 
Anjiphisbeena, i. 447, 448. 
Amphitrite, i. 281. 
Anabas, ii. 306. 
Analogy, Jaw of, i.49 ; ii. 625. 
Anarrhichas, ii. 128. 
Anchylosis, i. 382. 
Ancillatia, i. 241. 
Anemone, sea, i. 198. 
Angler, i. 422 ; ii. 390. 
Anguis, i. 447,. 454. 
Animal functions, i. 39. 
Anipoal orgauiz^ition, i. 96. 
Animalcules. See Infusoria. . 
Annelida, i. 269 ; ii. 249, 297, 

383, 479. 



Annular veflseb, L 74. 
AnodoD, i. 231. 
Ant, n. 386, 483, 486. 
Ant-eater, L 524 ; ii. 134. 
Antelope, ii. 147, 402. 
Antelope, horn of, i. 515. 
Antenns, i. 288 ; ii. 383. 
Antennolse, ii. 124. 
Anther, iL 596. 
Anthias, iL 306. 
Anthophora, i. 352. 
Antipsithes, i. 166. 
Antler of deer, i. 509. 
Antrum maxillare, ii. 400. 
Aorta, ii. 108, 609. 
Aphrodite, ii. 102, 125,298. 
Aplysia, ii. 126, 168, 551. 
Apodes, i. 423. 
Apterous insects, i. 296. 
Aquatic animals, i. 146. 
Aquatic plants, ii. 48. 
Aquatic tanrse, i. 309. 
Aquatic insects, i. 335. 
Aquatic birds, i. 592. 
Aquatic respiration, ii. 293. 
Aqueous humour, ii. 463. 
Arachnida, i. 282; ii. 121, 

316, 389, 485, 587. 
Aranea. See Spider. 
Arbor vitse, ii. 559. 
Arenicola, i. 277 ; ii. 295. 
Argonauta, i. 265. 
Aristotle f ii. 559. 
Aristotle, lantern of, ii. 119. 
Arm, human, i. 544. 
Armadillo, ii. 382. 
Arteries, i. 41 ; ii. 108. 
Articulata, i. 268. 
Ascaris, ii. 114, 540. 
Ascidia, i. 137; ii. 297. 
Ass, i. 516. 

Assimilation, i. 41 ; ii. 11. 
Astacus, ii. 435, 491. 
Asterias, i. 200; ii. 100, 208, 

235, 297, 383, 549, 586. 
Ateles, i. 399, 534. 
Atlas of Lion, i. 529. 

Atmosphere, porificalioii of, ii. 

Atmospheric lespiiatiaii, ii. 

Atriplex, ii. 48. 
AM^Umm, i. 290, 323, 324; 

iu 244, 317, 542, 
AudMbtm^ ii. 407. 
Auricle, ii. 108,259. 
Auricula, L 251. 
Avicula, i. 235. 
Axillae of plants, i. 90 ; iL 

Azelotl, ii. 324. 

Babiroussa, ii. 141. 
Bacculite, L 267. 
Ba«r,ii. 479,613, 634. 
Baker^ u. 478. 
Balsena. See Whale. 
Balance of affinities, ii. 7. 
Balistes, i. 432. 
Banksy i. 453. 
Barbels of fish, ii. 390. 
Bark, formation of, i. 86. 
Barnacle, i. 257 ; ii. 296. 
Bat, L 551; iL 136,567. 
Batrachia, i. 436 ; ii. 597, 
Batracbospermum, ii. 48« 
Bauer ^ i. 63. 
Bear, ii. 146. 
Beard of oyster, ii. 300. 
Beaver, i. 524; iL 149, 186, 

Bee, i. 351 ; iL 387. 
Belchier, i. 384. 
Bell (Sir C), u. 535. 
BeZZ (Thomas), L 482; ii. 409. 
Bellini, ii. 393. 
Berberis, i. 127. 
Berkeley, ii. 520. 
Beroe, i. 194, 203. 
Berzelius, iL 18. 
Bicuspid teeth, ii. 144« 
Bipes canalicttlatua, i. 457« 
Birds, L 554; ii. 130, 328, 

404, et passim* 




BliiMt*worm; i. 454, 457. 

Blood, ii. 334. 

Blood-vessels, ii. 281. 

Blumenbach, ii. 426. 

Boa, i. 447, 448. 

Boar, i. 56; ii. 141, 161. 

Bombyx, i. 300, 304, 312; 

ii. 486. 
Bone, i. 111,365,375. 
Bonnet, \. 53; ii. 17, 79, 92, 

252, 478. 
Barelli, i. 588. 
Boscy i. 149. 
Bostocky ii. 333. 
Bound of deer, i. 495. 
Bowerbanky ii. 241. 
Boyltj iL 16. 
Bractese, i. 94. 
BradypuA, i. 481 ; it. 284. 
Brain, i. 35; ii. 366, 555^ 

Brain, formation of, ii. 605. 
Branchiffi, ii. 267, 293, 299. 
Brassica, ii. 48, 53. 
Braula, ii. 483. 
Breschety ii, 427. 
Brewstery i. 232 ; ii. 472, 495. 
Brocken, spectre of, ii. 533. 
Bnmnannety ii. 587. 
Bruguiere, i. 149, 248. 
Bryophyllum, ii. 586. 
Buccinum, i. 215, 229, 242; 

ii. 126,301. 
Bucklandy ii. 206. 
Buds, i. 86 ; ii. 588. 
BuffoHy i. 185; ii. 530, 591. 
Bulb of hair, i.H 7. 
Bttlboffeather, i. 577. 
Bulbus arteriosus, ii. 273. 
Bulbulus glandulosus, ii. 185. 
Bulimus, i. 249. 
Bulla, ii. 168. 
Burrowing of the mole, i. 525. 

Cabbage, ii. 48, 53. 
Cachalot, i. 484 ; li. 142. 
Cseca, ii. 101,206. 

Ceecilia, ii. 497. 
Calamary, i. 261. 
Callionyrous, ii. 503. 
Calosoma, i. 320. 
Cambium, ii. 40. 
Camel, i. 108; ii. 176, 198. 
Cameleopard, i. 481, 498; ii. 

Camera obscura, ii. 458. 
Camerated shells, i. 265. 
Campanularia, ti. 234. 
Camper, ii. 437, 443, 561. 
Canada rat, ii. 178. 
Cancelli, i. 374. 
Cannon bone, i. 505. 
Capibara, ii. 160. 
Capillaries, ii. 263. 
Capsular ligaments, i. 106. 
Caput Med usee, i. 212. 
Carapace, i. 290, 463. 
Carbon, non-absorption of, ii. 

Carbonic acid, ii. 30, 337. 
Cardia, ii. 182. 
Cardium, i. 131, 221, 222, 

Carduus, i. 127. 
Carinated sternum, i. 566. 
CarlisUy i. 426, 434 ; ii. 286, 

Carnivora, i. 528 ; ii. 66, 145. 
Carp, i. 411, 429. 
Carpus, i. 405. 
Cartilage, i. 109. 
Caruncle, lacrymal, ii. 468. 
Carusy i. 366; ii. 208, 219, 

240, 252, 505. 
Cassowary, i. 586 ; ii. 224. 
Cat, ii. 392, 505. 
Caterpillar, i. 305, 315 ; ii.484. 
Caudal vertebrffi, i. 404. 
Cavolini, i. 159. 
Celandine, ii. 48. 
Cells of plants, i. 66, 69. 
Cellular texture, animal, i. 99. 
Centaurea, i. 127. 
Cephalic ganglion, ii. 541. 



Cephalo-thoraz, i. 282. 
Cephalopoda, i. 258 ; ii. 220, 

Cerambyx, i. dliS; ii. 311, 

Ceicaria, 1.186; iL 479. 
Cerebdliun, ii. 555. 
Cerebral ganglioD, iL 541. 
Cerebral hemispheres, ii. 556. 
Cerithhim, u 249. 
Ceroxyloo, iL 48. 
Cetaoea, i. 481,482; iL 142» 

176, 193,442,555, 6ia 
Ckabrier, L 108, 346. 
Chain of being, i. 53 ; ii. 629. 
Chalcides, i. 448, 457. 
Chameleon, i. 462; iL 129, 

390, 499. 
Chara, ii. 50,. 254. 
Chelidonian, ii. 48. 
Chelonia, L 463; ii. 130^276, 

321, 439. 
Chemistry, organic,. ii«'5y,33d. 
Cheselden, ii. 520. 
Ckevretdii i. 123. 
Children, u SIS; iL 491. 
Chitine, L 318. 
Chladm, ii. 4^7. 
ChondriUa, ii. 52. 
Choroid coat, ii. 462. 
Choroid ^land^ ii. 495. 
Chromatic aberration, ii. 474. 
Chromule, i. 70. 
Chrysalis, i. 307. 
Chyle, iL 107, 203. 
Chyme, ii. 181. 
Cicada, i. 340. 
Cicindela, ii. 2l2. 
Caia, L 126, 154, 157, 173^ 

Ciliary ligament, ii. 463. 
Cimbex, i. 333. 
Cimex, ii. 124. 
Cineritious, ii. 561. 
Circulation, ii. 11,229. 
Cirrhi, ii. 296, 389. 
Cirrhopoda, L 257. 

ClassitoOion, i. 51 ; n. G25. 

Clansilia, ii. 317. 

Clausium, i. 253. 

Clavicle, i. 404, 523, 566. 

Claviger, ii. 483. 

Chiw in lion's tatt, L 531. 

Clio, L 258; iL 138. 

Cioquet, ii. 498. 

Clypeasiar, L 21 U 

Cobitis, u. 309. 

Cobra de capello, i. 549; iu 

Coccygeal bone, i. 404. 
Cochlea, ii. 427. 
Cockchaffen SeeM^MoofhM. 
Cockle, L 221. &eCardiiim. 
Cod, lens of, L 59^ ii 4d& 
Coenurus, ii. 84.- 
Co-ejKis8ence of forms, i. 50. 
Coffin-bone, i. 517. 
Coleoptera, i. 348 ; tL 3d2. 
Collaii»bone, i. 404. 
Coiours of insects,!. 318. 
Colours, perceptions of, iL,5dl. 
Coluber, L 448,450; iL 1641 
Columella, i. 243 ; iL 439. 
Coaunissures of brain, ii« 5§21 
Comparettiy ii. 244, 436. 
Complementary goIooib). ii. 

Compounds eyes, ii«.48d. 
Concha of the ear, iL 42i. 
Condor, ii. 331. 
Conger eel, ii. 556. 
Conglomerate Ofes, iL 483, 
Conjunctiva, iL 466.. 
Consumption* of animai nuie^ 

ter, ii.,60. 
Contractility,, mnsoiilar, L 126. 
Coous^ i. 250. 
Convolutions of the fanuny ii. 

Convolvulus, iii.4A. 
Cooper J ii. 434. 
Coracoid bone, L 404^566. 
Coral, L 166. 
Coral island^ i. 15. 


Gorium, i. 112. 
Cornea, ii. 461.. 
Coraeule^.ii. 487. • 
Como Ammonig, K 267v 
Ck>roDet»boDe^.ii 517^ 
Corpora quadrigtfnim^.ii. 555. 
Coi|>\^ cidiDauin, iL 66.2. 
Corpus papillare^. ii* 378. 
Cortical substance, ii» 561. 
Cossua, i. 30Q« 312^,365. 
Cotunnius, ii. .427.. . 
Olowrie i 247 
Crab, i! 290 ; ii. 258; 2994 3 L7, 

Cranimn, i. 399, 400, 443, 

Craniam oi iaseclia^ u 322. 
Cmw, ii. 169. 
Cray-fisb,ii» 435,491. 
Cribrifomi plate, ii. 400. 
Crinoidea, i. 212. 
CrocQdile, i. 458^ 460; 462; 

ii. 142, 163, 276, 409, 440, 

Crop, ii. 179. 
Cross-bill, ii. 131. 
Crotalus, i. 450. 
Crust, i. 1 U, 292. 
Cruata pelrosa, ii^ 152^. 
Crustacea, i. 286; ii. 269, 

295,299,542,587. . 
Cryptogamift,.i; 71.; ii«.5^ 
Crystalline lens, i. 59<; ii; 

Crystalline needles in biliary 

ducts, ii..2d9.. 
Curculio, i. 328. 
Coshiooa of insects, i* 331. 
Cuticle, vegetable, i. 77. 
Cuticle, animal, i. 1.12[; ii. 

Cutt]$r4sh. Sm' Sepia. 
CuvUry pawBb 
Cuvier (F.), i, 1:20, 574, 
Cyclidiu, i. 186. 
Cycloceela, ii. 98. 
Cyclosis, ii. 49, 233. 

Cyclostomata, ii. 116. 
Cyinbia, i. 241. 
Cymothoa, ii. 544; 
CyprsBa, i. 5&ti7; 
Cyprinus, i..llj6, 4ii. 
Cysticule, ii. 4%. . 

Daldarff, i. 433 ; ii. 306. 

Darwin^ i. 89. 

Darwin (Dr. E.), ii. 530i 

Davy, ii. 17, 338i 

I>ary-(Dr.), ii. 274* 

Deatb, ii. 624. 

De BUdnmlle, i. 63, 248, 366; 

ii. 252, 428, 482, 497, 570; 
De CandoUe, i. 93 ; iii 19^25, 

28, 30, 38, 5i, 620i 
De CaiuMle (junior), iii 471 
Docapoda, ii. 258» 
Decline of the system, ii. 619. 
DecoUated'diell*, i. 249« 
Deer, i. 507-, ii. 402. 
Defrance^ i. 256. 
De Qeety i. 341. 
Deglutition, ii^ 174. 
Delaroche, ii. 309, 497. 
De Mont^re, ii: 183. 
Dormo-skdeton, u 366^ 
De Saussure (Th.), ii. 30. 
Des Cartes, ii. 364, 560. 
De Serres, ii. 211, 239, 485. 
Design, eiridence of, il 28. 
Design, unity o^ ii. 625. 
Developement, vegetable, i. 8^ 
Developement, animal, ii. 599i 
Diaphragm, ii. 326, 61 1 . 
Diffusion of animalsi, ii. 64;, 
Digestion, i. 41 ; ii. 180. 
Digitigrada, i. 533. 
Dioecia, ii. 596. 
Dionsea, i. 128. 
DiplozQon, ii. 608. 
Diptera, i. 323, 353; ik 115. 
Diquemare, i. 220« . 
Distoma, ii. 1 1^. 
Divisibility of matter^ ii. 397» 



DolUnget, ii. 614. 
Dolpyn, ii. 142, 442,507,616. 
Doras costatOB, ii. 307. 
D'Orhignyy i. 265. 
DorU, ii. 126, 296. 
Dormoose, ii. 191. 
Dorsal vessel, ii. 236. 
Dory, i. 421. 
Dove, ii. 554, 557. 
Down of plants, i. 94. 
Down of birds, i. 572. 
Draco volans, i. 56, 547. 
Dragon-fly, i. 310, 351 ; ii. 

Dreaming, ii. 536. 
Dromedary, ii. 223. 
Duckweed, ii. 589. 
Dufour (Uou), ii. 209, 313. 
Dugh, ii. 244, 250, 479, 487, 

bugong, ii. 142, 279, 442. 
Duhamel/\\. 16,20. 
Dumas, ii. 393. . 
Dumhil, ii. 411. 
Dumartier, u 366. 
Duodenum, ii. 208. 
Dutrochet, i. 75, 190; ii3l4. 
Dytiscus, i. 29, 311, 333, 336 ; 

ii. 311,313. 

Eagle, ii. 130. 

Ear, ii. 421. 

Ear-drum, ii. 422. 

Earh, i. 560. 

Earths in plants, ii. 43. 

Earth-worm, (see Lumbricus). 

Echinodermata, i. 199. 

Echinus, i. 203»210; ii. 101, 

Edwards, ii. 317, 542, 631. 
Eel, i. 424 ; ii. 307. 
Egg, ii. 597. 
Ekrenberg, i. 13, 186, 189; 

ii. 93, 478, 592. 
Ehrmann, ii. 309. 
Elaboration,' successive, ii. 13. 
Elastic ligaments, i. 107. 

Elater, 1.341. 
Elearine, i. 123. 
Electric organs,'i. 31 « 
Electricity, ii. 350, 
Elements, organic, it. 6. 
Elephant, i. 56, 108, 491, 518 ; 

ii. 141, 154, 162, 199, 2!», 

392, 504, 569. 
Ellis, i. 150. 

Elytra, analysis of, L 318, 349. 
Embryo, ii. 595. 
Emu, i. 586. 
Emys, i. 474. 
Enamel of teeth, ii. 150. 
Endogenous plants, i. 83. 
Entomoline, i. 115, 318. 
Entomostraca, ii. 493. 
Entozoa, i. 282; ii. 83, 113, 

Ephemera, i. 311 ; ii. 241. 
Epidermis, vegetable, i. 88. 
Epidermis, animal, i. 1 12, 1 13, 

Epiphragma, i. 253. 
Equivocal genention, ii. 591. 
Equorea, ii. 85. 
Erato, i. 247. 
Erect vision, ii. 521. 
Erpobdella, i. 272 ; it. 252. 
Eryx, i. 447. 
Esox, i. 427. 
Ethmoid bone, ii. 400. 
Eudora, ii. 91. 
Euler, ii. 475. 
Eunice, ii. 480. 
Euphorbium, ii. 59. 
Euryale, i. 212. 
Eustachian tube, ii. 424. 
Evil from animal warfare, i. 4^; 

ii. 67. 
Excretion, ii. 12. 
Excretion, vegetable, ii. 46, 51 • 
Exhalation by leaves, ii« 27. 
Exocettts, i. 547. 
Exogenous plants, i. 83. 
Eye, i. 31; ii. 460, 587, 589. 
Eye, formation of, it. 605. 



Eye-lidSy fonnatioii of, iL 615; 

FahriciuSy i. 195. . 

Facial angle, ii. 561. 

Fairy rings, ii. 55* 

Fallacies of perception, ii. 514. 

Fangs of serpents, ii. 163. 

Fdrtidayj ii. 524. 

Fasciola, ii. 113. 

Fasciolaria, i. 249. 

Fat, i. 123. 

Fata Morgana, ii. 533. 

Feathers, i. 568, 591. 

Fecula, i. 70. 

Fecundation, ii. 595. 

Feelers, i. 288 ; ii. 383. 

Feet-jaws; i. 289. 

Feet of birds, i. 584. 

Femur, i. 287, 328, 405. 

Fenestra of ear,.ii. 425. 

Ferns, i. 83 ; ii. 593. 

Fibre, animaJ, i. 98, 105. 

Fibula, i. 405. 

Fig-tree, ii. 48. . 

Fig Marygold, ii. 48. 

Filaments of feathers, i. 569. 

FUaria, i. 63. 

FQices, i. 83 ; ii. 593. 

Final causes, i. 1 ,22, et passim. 

Fins of fishes, i. 421. 

Fins of cetacea, i. 486. 

Fishes, i. 109, 408; u. 127, 
272, 389, 410, 494, et pas- 

Fissiparous reproduction, ii. 

Flea, i. 297. 

Flight, i. 344, 545. 

Flourens^ ii. 305. 

Flower, ii. 595. . 

Fluidity, organic, i. 61. 

Flustra, i. 165, 169, 172. 

Flying fish, i. 547. 

Flying lizard, i. 547. 

Flying sqiurrel, i. 550. 

Focus, ii. 453. 

Fohmann, \k 353. 

Follicles, i. 114; ii. 185. 
Fontana^ ii. 614. 
Food of plants, ii. 15. 
Food of animals, ii. 57. 
Foot of mollusca, i. 221. 
Forces, physical, i. 6. 
Fordyce, ii. 172, . 
Fovilia, ii. 596. 
French bean, ii. 52. 
Frog, i. 437; Ii. 128,222,274, 

Fucus yesiculostts, i. 66. 
Functions, i. 34, 38 ; ii. 69. 
Fungi, ii. 55, 
Furcular bone, i. 506, 
Farcularia, i. 62. 
Fusiform roots; ii. 21. 
Future existence, ii. 580, 640i 

Gaede^ ii. 86. 
Gaimard, i. 97. 
Graleopithecus, i. 550. 
Galileo, i. 81. 
Gallinee, ii. 554. 
Gallop, i. 495. 
Galvanism, ii. 514. 
Ganglion, ii. 358. 
Gasteropoda, i. 227; ii. 176, 

Gastric juice, ii. 183. 
Gastric teeth, ii. 167, 214. 
Grastric glands, ii. 184. 
Gastrobranchus, i. 407, 416 ; 

ii. 116,497. 
Gay Lussacj ii. 314. . 
Gecko, i.4fia; ii. 390. 
Gelatin, i. 105. 
Gemmiparous reproduction, ih 

Gemmule, i. 156 ; ii. 591. 
Greometer caterpillars, i. 315. 
Grerms, vegetable, i. 86 ; ii^ 

Geronia, ii. 91. 
Gillaroo trout, ii. 202. , 
Gills, i. 439 ; ii. 267, 299. 
Gimbals, i. 330. 



Gizzard^ ti. 169, 214. 
GrlandSy ?egetabk, i. 77; iu 

Glands, animal, ii. 34B. 
Glands in dooodile, ii. 409. 
Glands, gastric, iL 184. 
Oleichen^ ii. 94» 
Globules, i. 64, 98. 
Glossa, ii. 124. 
Glossopora, iL 104. 
Omeliuy i. 149. 
Gnat, ii. 115. 
Gottt, ii. 402. 
OoezCf ii. 478. 
Gonittm,i. 187. 
Goose, ii. 170, 500. 
Gordias, i. 63» 276. 
Gorgonia, i« 166. 
Gradation of being, i. 53 ; ii. 

Grampus, ii. 142. 
Grallse, i. 585, 592. 
Grant, i. 147, 151, 169v 172, 

175, 195, 203, 215, 587 : ii. 

Gray, i. 219, 239, 254. 
Growth, vegetable, i. 84; ii. 

Gruithuiten, ii. 479. 
GrylloUlpa, i. 342 ; ii. 385. 
GryUus,.ij. 244. 
Guinea-pig, i..498i. 
Gulstoniaii lectures, ti. 532. 
Gum, ii. 37. 
Gurnard, ii» 554. 
Gymnotus, i. 424 ; ii. 572. 

Heematopus, ii. 13K 
Haidinger, i. 205. 
Hair, iFegetable, i. 94.. 
Hair, animal, i. 117, 31^. 
Hair-worm, i. 276. 
Hales, ii. 26. 
Haliotis, i. 231. 
Nailer, i. 98. 
Halteres,!. 353. 
Hamster, ii. 178. 

Hmacaek^ ii. 307. 

Hand, i. 544 ; iL 392. 

Hanawy iL 478. 

Hare, L 497 ; iL 149, 191. 

Hartley, n. 563. 

Hiuwood, ii. ^M, 405. 

Hatchett, u. 43. 

HoMktbee, ii..416. 

Hannch in insects, L 287, 32& 

Hawk, iL 130. , 

Head of insects, i. 322. 

Hearing, iL 414, 571. 

Heart, L 41, 138; iL 258,6tt7L 

Hedge-hog, i. 524, 527. 

Hedysanim gyrans, L ld7i 

Hedwifft L 74. 

Helix, L 242, 253; ii. 12&, 

Hellman, ii. 390. 
Hemipte»> i. 309, 360; iii 

Hemispheres, cerebral, iti. 556. 
Henbane, iL 59... 
Henderson, ii. 338. 
Hepatic vessels, ii. 208^ 214. 
Herring, L 42L. 
Herschel (Sir W.), ii. 529. 
Herschel (Sir John), i. 2d8 ; 

Hervey, ii. 288. 
Hesperia, i^ 356^ 
Hexastoma> ii. 113* 
Hippopotamus ii. \ASLj 151, 

152, 162, 193, 443, 504i 
Hirudo, L 138^ 281 ; iL y», 

Hodgkin, i. 99, 127. 
Hodgson, iL:403. 
Hog, i. 402, 521 ; u. 19i3, 392. 
Holothuria, ii. 208, 235, 296, 

Home (Sir Everard), passim* 
Honey-comb storaadi, iL 195. 
Hooded snake, L 549^ 
Hooks on filet of insects, L 331 . 
Hop, i. 91. 
Horn, i. 115,514. 



Hom on beakof cbick, ii..616. 
HoTse, K 516^ u. 191, 401, 

Horse-fly, ii. 115.. 
Hostilities of animals, i. 46 ^ ii. 

Houstan^M, 129. 
ir«^,ii. 386,413. 
Human fobric, i. 536 ; ii* 559. 
Humboldt, ii. 308, 314^ 338. 
Humerus, i. 405*. 
Hmnounrof Uie.eye^ ii; 460. 
Httntevi i..l08; u. 171, L8fi, 

Hysna, i. 499; u.*6U 149« 
HybematioD, ii. 536^ 
Hydatid, u. 84,113, 591. 
HydaUna, ii. 97, 98, 479^ 539. 
Hydra,, i. 162,. 176; u. 74, 

477,. 538, 586, 590. 
Hydrogen, ii. 45. 
Hydrophilut^ i. 3il, 
Hydrostatic acalephft, i^ 196. 
Hyla, i. 445. 
Hymenoptera, i..32d, 351; ii. 

1 16 244. 
Hyoid'bone, ii. 132, 303. 
Hyrax, ii. 191. 

Ichthyosaurus, i. 469.^ 
Ilium, i. 405. 
Imago, i. 307, 317.. 
Incisions of insects, i. 327. 
Incisor teeth, ii. 143. 
Incus, ii. 426.^ 
Indian walrus, ii. 142. 
Individuality of polypes, i. 173. 
Infusoria, u 183; ii. 539,583. 
Illplrie9^ repamtion of, ii. 3, 

Inorganic world, i. 7. 
Insects, i. 11, 108, 296; ii. 

207, 236,. 395, 436, 546, 

lasectifora,. L. 525. 
Instinct, ii. 574^ 
Integuments, i. HI ; ii. 377. 

Intercellulac spaces, i. 70. 
Intermaxiltary^ hone, ii. 142^ 

Interspiooua bones, . i. 396; 
Intestine, ii. 101. 
Iriartea, ii. 48« 
Iridescence, i..232.. 
Iris, i. 136; ii. 463. 
Ischium, i. 405. 
Isis, i. 168^ 
Ivy, i.. 92. 

Jacobson, ii. 568,. 570. 
Jerboa, i. 497,538. 

Julus, i. 298 ; u. 485. 
Jurine"^ ii. 567.. 

Kaleidoscope, ii. 533. 
Kanguroo,. i.. 399,^ 497,. 538 ; 

ii. 193, 598. • 
Kater, ii. 491. 
Kerona,^ i. 186. 
Kidd, i. 342; iu 313, 348, 

Kieman, ii. 350. 
Kieser, i. 66, 74.. 
^t%, i. 327; ii 413^485. 
Knighty ii. 595. 
Knots in wood, ii. 589. 
Koala, i. 527. 
Koipoda, i. 187. 

Labium of in8ftct8,.iu 123. 
Lahrum of inacct8,.ii. 123. 
Labyrinth, ii. 427. 
Lacerta, i. 457, 458. 
Lacrymal organs, ii. 466. 
Lacteals, ii. 107, 226. 
Lamarck, u 149 ;: u. 93,637. 
Lamina spisalia, ii. 430^ 
Lamouroux, i. 149.. 
Lamprey, i. 416; iL 116,305, 

Lancets of. di|9tera, ii.. 115. 
Language of msects,. ii.. 386. 
L«k^ i..5a2; 



Lanrm, L 304, 306. 
LoMsai^fme, L 318 ; iL 183. 
Lathom^ iL 189. 
LaireUU, i. 290 ; ii. 316, 389, 

Lawi of nature, L 6. 
Law of mortality, t. 42. 
Law of co-eziflteace of fixms, 

i. 50. 
Law of gradation, ii. 629. 
Law of analc^, t. 49 ; ii. 625. 
Leach, i. 219. 
Leaves, ii. 29, 44. 
Leech, (see-Hinido). 
Lemar, L 533, 550; ii. 285, 

Lens, cryBtalline, i. 59; ii. 

462, 496. 
Lenticellae, i. 93. 
Lepaa, i. 257 ; ii. 296. 
Lepidoptera, L 304, 354; ii. 

Lepisma, i. 297, 298, 356. 
Lemaea, i. 302 ; ii. 600, 608. 
Leuchs, ii. 482. 
Leucophra, ii. 96. 
Leurety ii. 183. 
Lewenkoeckf i. 356 ; ii. 264. 
Libellala, i. 310, 351; ii.486, 

Liber, i. 88 ; ii. 41. 
lichen, ii. 19. 
Life, i. 34, 42. 
Ligaments, i. 106. 
Ligamentum nuchse, i. 108, 

Light on plants, i. 91 ; ii. 28. 
Lignin, i. 70; ii. 41. 
Lilium, i. 78. 
Limax, u. 126,317. 
Limpet, (see Patella). 
Link, i. 75. 
Lion, i. 108, 496, 529 ; ii. 136, 

392, 557. 
Lister, ii. 233, 300. 
Liver, u. 219, 350. 
Lizard, ii. 129, 390, 497, 587. 

Lobster, L 293; iL 167, 958, 

299, 435, 544. 
Lobdaria, L 161. 
Loche, ii. 300* 
Loooosotioii, L 143. 
LocQSta, ii. 122. 
Loligo, L 261, 407; iL 271. 
Longevitj of trees, ii. 620. 
Lophins, L422; n. 390, 437. 
Lozia, ii. 131. 
Locanos, L 359. 
Lombricos Bsarniiis, i. 277,295* 
Lnmbficns t eri e sti i s , iL 102« 

1 14, 254, 297. 
Langs, iL 267, 611. 
Lycopodiom, L 78. 
Lycoris, ii. 480. 
Lymphatics, iL 352. 
Lymphatic hearts, ii. 353. 
Lyanet, L 300, 312, 365. 

Macam, ii. 51, 54, 58, 334. 
Macartne^^ L 590; n. 329, 

331, 562. 
Macavo^, ii. 375. 
Mackerel, i. 425. 
Macleay^ i. 54. 
Madder, i. 384. 
Madrepore, i. 166. 
Magendie, U. ,$05, 635. 
Magihis, L 249. 
Maia, u. 269, 645. 
Malleus, iL 426. 
Malpighi, iL 378. 
Mammee, ii. 598, 616. 
Mammalia, i. 477; iL 326, 

Man, i. 536 ; ii. 559. 
Man of war, Portagese, i. 196* 
Manatus, ii. 142. 
Mandible, i. 289. 
Mantis, iL 211. 
Mantle, L 113, 237. 
Many-plies stomach, ii. 197. 
Marcet, ii. 58, 226, 334, 338. 
Maiginella, i. 247. • 
Marmot, ii.-149. 



Mariigliy i. 150. 
Marsupialia^ ii. 277,598. 
Marsupium, ii. 500. 
Mastication, ii. 140. 
Mastoid cells, ii. 425. 
Matrix of feather, i. 576. 
Matter, ii. 516, 
MaunxriTy ir. 527. ' 

Maxlllffi, ii. 123. 
MayeVf i. 447. 
MayOy ii. 618. 
Meatus auditorius, ii. 422. 
Mechanical fdnct&oas, i. 38« 
Meckel, i. 482 ; ii. 480. 
Medulla oblongata, ii, 555, 
Medullary substance, ii. 365. 
Medullary rays, i. 86. 
Medusa, i. 96, 192 ; ii. 63, 72, 

85, 294, 478. 
Meibomian glands, ii. 469. 
Melolontha, i. 300; ii. 122, 

212, 236, 313, 486, 490. 
Melophagus, ii. 483. 
Membrana nictitans, ii. 499, 

Membrane, i. 101. 
Menobranchus, ii. 324. 
Mercurialis, ii. 53. 
Mergus, ii. 130. 
Merrythought of fowl, i. 566. 
Mesembryanthemum, ii. 48. 
Mesenteric glands, ii. 227. 
Mesentery, ii. 108. . 
Meso thorax, i. 323. 
Metacarpus, i. 405. 
Metals in plants, ii. 43. 
Metamorphoses, i. 302, 437 ; 

ii. 632, 634. 
Metatarsus, u 405. 
Metathorax, i. 323. 
Milk, ii. 616. 
Millepedes, ii. 485. 
Millepora, i. 167. 
Mimosa, i. 127. 
Mint, ii. 17, 30. 
Mirandolay ii. 586. 
Mirbely i. 69, 72. 

Mite, i. 297. 
Mitra, i. 248. 
Modiolus, ii. 431. 
Molar teeth, ii. 144. 
MoldenkaweTy i. 74. 
Mole, i. 524, 525 ; ii. 391 , 505. 
Mole cricket, i. 342. . 
MoUusca, i. 213; ii. 269, 389, 

Monas, i. 13, 184; ii. 96, 583. 
Monkey, i. 533 ; il. 149, 392, 

Monoculus, ii. 493. 
JVIonothalamous shell, i. 265. 
Monotremata, ii. 277. 
M<mrd, i. 123, 132; ii. 303. 
Mordella, ii. 486. 
Morpho, i. 354. 
Morren, ji. 252, 255. 
Mortality, i; 42 ; ii. 581. 
Mother of pearl, i. 232. 
Motion, voluntary,, i. 37; ii. 

Motion, vegetable, i. 127. 
Motor nerves, ii. 535. 
Mucous membrane, i. 112. 
Mucous glands, ii. 184. 
Mulberry, ii. 59. 
MuUery i. 183; ii. 92, 353, 

Mullet, ii. 202. 
Multiloc.ular shells, i. 265. 
Multivalves, i. 257. 
Mureena, ii. 497, 556. 
Murex, i. 245, 252; ii. 126, 

Mus, ii. 178,506. 
Musca, i. 332. 

Muscle (shell Esh), i. 222, 224. 
Muscle, i. 124, 127, 300. 
Muscles of eye; ii. 464. 
Muscular power in plants, ii. 

Muscular power in birds, i. 

Mushroom, ii. 19. 
Musk shrew, ii. 135. 



Musical tone, ii. 419. 
Mya» i. 223. 

Myriapoda, i. 297, ii. 048. 
Mynnecopha^, ii. 134. 
Mysis Fabncii, i. 289. 
Mytiliis, i.^222. 
Myxine, i. 407, 416; ii. 116, 

Nacreous structare, i. 231. 
Nai8,ii. 102,251,479,586. 
Narwhal, i. 56; ii. 14 h 
Nature, i. 6, 13. 
Natttilus, i. 242, 266 ; ii. 270. 
Necrophorus, ii. 413. 
Needles in biliary dncto, ii; 219. 
Nereis, i. 271, 274,280; «. 

Nerve, i. 36 ; ii. 366. 
Nervous system, ii. 365, 537, 

Nervous power, ii. 354. 
Nettle, u. 47. 
Nenro-skeleton, i. 366. 
Neuroptera, i. 851. 
Newport, i. 352 ; sL 102, ^14, 

Newt, ii. 439, 587. 
Nightshade, ii. 59. 
Nitrogen, ii. 14, 338. 
Nordmann, ii. 600, 608^ 
NotonecU, i. 29, 337. 
Nursling sap, ii. 24. 
Nutrition, li. 1, 10,13,57. 
Nutrition in lower orders/ ii. 74. 
Nutrition in higher orders, ii. 

NutriUve functions, i; 38. 
Nycteribia, ii. 483. 

Octopus,!. 261; ii. 494. 
Ocularspectra, ii. 530. 
Odier, i. 318. 

(Esophagus, ii. 101, 107, 176. 
Oken, i. 349, 400. 
Olfactory nerve, ii. 396. 
Olfactory k)bes, ii. 556. 

Olivse, 1.241, 250. 
Oniscus, ii. 544. 
Onocrotalus, i. 566. 
Operculum of Molbnoa, i. U2 . 
Operculum of fishes, ii. 303. 
Ophioephaius, ii. 307. 
Ophidia, i. 447. 
Ophiosaurus, i. 454, 457. 
Ophiura, i. 212. 
OpoNMum, ii. 136, 598. 
Optic axis, ii. 503. 
Optic ganglion, ii. 489. 
Optic lob^, ii. 565. 
Opuntia, i. 127. 
Orache, ii. 48. 
Orbicular bone, ii. 436. 
Orbicular muscle, i. 136. 
OrchidesB, i. 69. 
Oi^nic Mechanism, i. 50, 96. 
Organic developement, ii. 599. 
Ornithorhyncus^ i. 395; ii. 136, 

178, 391,442,497. 
Orobanche, ii. 54. 
OrthoceratJte, i. 267. 
Orthoptera, i. 309, 349. 
Os hyoides, ii. 132, 303. 
Osier, i. 206, 220, 283, ^77, 

Osseous fabric,' t. 365. 
Ossicula, tympanic, ii. 426. 
Ossification, i. 375, 566. 
Ostracion, i. 432. 
Ostrich, i. 563, 587, 590; ii. 

185, 224, 328, 554. 
Otter, sea, ii.'149. 
Ovary, ii. 593, 594. 
Oviduct, ii. 596. 
Oviparous animals, ii. 597. 
Ovo«vivipait>u8aniBals,ii. .597. 
Ovula, i. 247. 
Ovum, ii. 593. 
Owen, i. 563. 
Owl, ii. 330, 441,503. 
Ox, horn of, i. 515.- 
Oxygen, ii. 29. 
Oyster,!. 131,220,221. 
Oyster-catcher, ii. 131. 



Pac^sof quadrupeds, i. 492. 
PachydermaU, i. 618 ; ii. 382. 

Package of m^aiis, i. 102. 

PiJA, ii. 368. 

Palemon, ii. 644. 

Paley,\. 102,671 ; ii. 286. 

Palinunis, ii. 644. 

Pa/Aw, i..l60:; ii. 344* 

Palms, i. 83. 

Palm squirrel, ii. 178. 

Palmer^ ii. 30. 

Palj^i, 1,289; ii.:l24. 

Pancreas ii. 221. 

Pander, ii. 607. 

Pam[iiculus camoftus, i.»527. 

Panorpa, i. 326. 

Paper nautilus, i. 266. 

Papilio, i. 367 ; ii. 486. 

Papill8B,.ii. 378, 394. 

Par yagum, ii. .549. 

Parakeet, ii. 131. 

Parallax, aberration of, ii. 472. 

Parrot, ii. 179, 391. 

Pastern, i. 517. 

Patella, i. 228; ii. 220,661. 

Patella of knee,' i. 406. 

PirtcUarta, H. 46. . 

Pauneh, ii. .195. 
P«ccaci| ii. 193. 
Pediculus, i. 297. 
PeljttW, i. 666; ii. 178. 
Peiieil of cays, ii. 453. 
Penguin, i. 692. 
PenitentiHTjr, it 189. 
Penilatula, i. 174; ii. 82. 
PffeMiform tttuscle/ i. 133. 
Pentacriaus» i. 212. 
Perca, i. 11^, 4^; ii. 306, 
. -41 0» 496, 657. 
Perception, i. 36; ii. 372, 608. 
Perch (Sk^ Perca). 
Perennibmncbtay ii. 324. 

Pefilymfrfi,' «. 427. 
Periomticiitti, i; 237. 

Peristaltic motion, ii. 204. 
Peron, L 97.; ii. 72. 

P/a#, ii. 338. 
Phalanges i. 406. 
Phalena, ii. 244, 486. 
Phanerogamous plants, ii. 595. 
Phantasmagoria, ii. 533^ 

Phaseblus, ii. 52. 

Phenakistioope, ii. 524. 
Philip, ii. 190, 360. 
Phoca, i. 487. 
Pholas, i. 220, 266. 
Phosplioreacence of tbe sea, 

i. 194; ii. 63. 
Phrenology, ii. 585. 
Phyllosoma, ii.,544. 
Physalia, i. 196* • 
Physiology, i. 21, 
Physsophoica, i. 197. 
Phytozoa, i. 146. 
Picrorrf, -ii. 200. 
Pigeon, ii. 179, 616: 
Pigmedtum.ofskin, i; 112. 
Pigmentun^of the eye, ii. 462. 
Pike, 1.487. 
Pileopsis; i. 252. 
Pineal gland,, ii. 560. 
Pinna, i. 224, 235. . 
Pistil, ii. 696. 
Pith of plants,! 85.. 
Fith of quill, i. 680. 
Placuna, i. 230. 
Planaria, ii. 114, ,236, 250, 

294, 479, 586. 
Planorbis, i. 227, 242. 
Plastron, i. 463. 

Pleurobrancbus, ii«.220. 
Pleurimectes,.!. 431 > Ii. 5C*3. 
Plexus, nervous, li. 359. 
Pliny ;^. 569. 
Plumula, ii..603. 
Plumularia, ii. 234. 
Pneumo-branchisB, ii.'316, 

Pneumo^gastric- nerve, ii. 5^9. 
Podura, i. 297. 



Fomea, L 353. 

PoHOD of nettle, ii. 47. 

Poii, L »7, 235. 

PjoOen, ii. 596. 

Poljgaftnca, ii. 97. 

Poijps L 161 ; iL 74, 81, 2d3, 

PoljsUMBa, ii. 113. 
PolytfaalaoMNH theO, i. 265. 
Pontia hrmnci, L 354. 
PoDtobddla, L 271. 
Poppy, iL 48. 
Po r c upin e quills, i. 120. 
Ponrupine, i. 527 ; iL 149, 193. 
Porilera, L 147. 
Porpita, i. 195. 
Porpus, ii. 142, 193. 
Porter6eld, i. 374. 
PoUtoe« ii. 589. 
Prehensicm of food,ii. 1 1 3, 1 17. 
Priestley, ii. 29, 336, 338. 
Prittis, L 56; ii. 166. 
PrUckard, ii. 241. 
Priret Hawk moth, ii. 218. 
Probotcis of insects, ii. 114. 
Proboscis of moUusca, ii. 126. 
Proboscis of Elephant, i. 520. 
ProgressiYe motion, L 144. 
ProUigs, i. 313. 
Promontory of ear, ii. 425. 
Proteus, i. 187 ; ii. 324,632. 
Prothorax, i. 323. 
Praut, ii. 37, 41. 
Provencal , li. 308. 
Proximate principles, ii. 6. 
Pterocera, u 246. 
Pteropoda, i. 257. 
Pteropus, ii. 136. 
Pubic bone, L 405. 
Pulmonary organs, ii. 267. 
Puncta lacrymalia, ii. 468. 
Punctnm saliens, ii. 607. 
Pupa, i. 304, 307. 
Pupil, ii. 463. 
Pupipara, ii. 483. 
Pyloric appendices, ii. 221. 
Pylorus, ii. 107, 182. 



iL 500u 
i,L533; iL149. 
Quadfpc da , L 487. 
Qnagga, L 516. 
Quail, L 582. 

Quills of poicapine, L 120. 
Quills of Icathes^ L 568. 
Qwiy, L97. 

Rabbit, L 497; iL 149,190. 

Racoon, L 112. 

Radiata, L 164. 

Radides, iL 603. 

Radius, i. 405. 

Ranunculus, i. 79. 

Ropp^ n. 478. 

Rat, ii. 148, 149, 192. 

Raikke, iL 634. 

Rattle-snake, L 450. 

i2ay,L 11. 

Ray, L 420, 422, 423 ; fi. 503, 

Rays of fins; i. 424. 
Razor-shell-fish, L 222. 
ReauwMr, L 199, 202, 227, 

237,292; iL 115,170, 183. 
Receptacles of food, ii. 178. 
Receptaculum chyli, iL 108, 

Reed of ruminants, ii. 197. 
Refraction, law of, ii. 453. 
Regeneration of claw, i. 295. 
Rennet, ii. 197. 

tion, ii. 3, 9, 587. 
tion of organs, L 57. 
Reproduction, L 43 ; ii. 581 . 
Rei^taes, L 435 ; ii. 273. 
Resinons secietioDB, ii. 47. 
Respiration, L 41 ; ii. 11, 265, 

Rete muscosnm, i. 112. 
Reticulated cells, i. 69. 
Reticule of Ruminants, ii, 195. 
Retina, ii. 374, 448, 462. 



RetUFDiDg sap, ii* 36. 
Revelation, ii. 641. 
Reviviscence, i. 62 ; ii. 255, 
Rhea, i. 586. 
Rhinoceros, i. 515; ii. 135, 

Rhipiptera, i. 350. 
Rhizostoma, ii. 87. 
Rhyncops, ii. 132. 
Ribs, i. 401 ; ii. 327. 
Ricinus, i. 297. 
Rings of annelida, i. 272. 
Rodentia, i. 523 ; ii. 148, 151, 

162, 175, 191,504. 
Roesel, ii. 478. 
Roget, ii. 9, 524, 532, 582. 
Rolando, ii. 613. 
Roosting, i. 588. 
Roots, i. 93 ; ii. 20. 
Ross^ i. 16. 
Rostrum, ii. 124. 
Rotifer, i. 62, 189; ii.92,479, 

539, 591. 
RouXi ii. 569. 
Rudimental oigans, i. 55 \ ii. 

Rudolphiy i. 75. 
Rumfvrdf i. 76. 
Ruminantia, i. 499; ii. 196, 

Ruscofdf ii. 613. 

Sabella, i. 277. 
Sacculus of ear, ii. 430. 
Sacium, 404. 
St. Ange, ii. 277. 
' St. HilairCf passim. 
Salamander, i. 446 ; ii. 128, 

498, 597. 
Salicaria, ii. 54. 
Saline substances in plants, ii. 

Saliva, ii. 175. 
Salmon, ii. 222. 
Sand-hopper, ii. 542. 
Sap, ii. 24. 
Sauria, i. 457 ; ii. 276. 


Savigny, i. 274, 290; ii. 119, 

Saw-fish, ii. 166. 
Scala tympani et vestibiuli, ii. 

Scales of lepidoptera, i, 354. 
Scales of fishes, i. 116. 
Scansores, i. 586 ; ii. 554. 
Scapula, i. 404. 
Scarabeeus, ii. 486. 
Scarf skin, i. 112. 
Scarpa, I 101; ii. 411,430. 
Schceffevy ii. 478. 
Schneiderian membrane, ii. 

SchultZy ii. 49. 
Sciurus, i. 550; ii. 178. 
Sclerotica, iL 460. 
Scolopendra, i. 298; ii. 248, 

Storeshy, i. 194. 
Scorpion, ii. 315, 485. 
Scuta, abdominal, i. 453. 
Scutella, i. 211. 
Scyllffia, i. 229. 
Sea, phosphorescence of, i. 194 ; 

ii. 63. 
Sea-hare, ii. 126, 168, 551. 
Sea-mouse, ii. 102, 125, 298. 
Sea-otter, ii. 149. 
Seal, i. 487; ii. 403, 442, 506. 
Sebaceous follicles, i. 114. 
Secretion, ii. 12, 45, 342. 
Seed, ii. 593. 

Segments of insects, S. 320. 
Sembiis, ii. 242. 
Semicircular canals, ii. 427. 
Senecio, ii. 53. 
Sennehkr, ii. 20, 29. 
Sensation, ii. 362. 
Sensibility, variationsof, ii. 526. 
Sensitive plant, i. 127. 
Sensorial power, ii. 360. 
Sensorium, ii. 508. 
Sepia, i. 261 ; ii. 126, 203, 

Seps, i. 458. . 

u u 



Series of organic beings, i'. 5^ 
Serous membranes, i. 10'2. 
Serpents, i. 447 ; ii. 129, 163, 

Serpula, i. 277 ; ii. 295. 
Serres, ii. 609, 617. 
Sertularia, i. 165; ii. 234. 
Semm, i. 102. 
Sesamoid bones, i. 406. 
SetiB i. 274. 
Shark, ii. 162, 205, 262, 495, 

569, 587, 598. 
Sheep, u. 153, 194, 402. 
Shelly i. 111,230. 
Sheltopusic, i. 457. 
Shrapnell, ii. 426. 
Shrew, u. 149,391. 
Shuttle bone, i. 517. 
Silica, ii. 18,44. 
Silk worm, i. 305 ; ii. 59. 
Silurus, ii. 307, 390. 
Sinistral shells, i. 243. 
Siphonaria, i. 252. 
Siren, i. 457; ii. 324. 
Skate, ii. 303, 410, 495. 
Skeleton, 1. 365, 386. 
Skeleton, vegetable, i. 95 ; ii. 

Skimmer, ii. 132. 
Skin, ii. 377. 
Skull (see Cranium). 
Slacky i. 66. 
Sleep, ii. 536. 

Slips, propagation by, ii. 585. 
Sloth, 1. 481 , 498, 524 ; ii. 284. 
Slug, ii. 126, 317. 
Smell, ii. 396. 
Smith, ii. 1 66. 
Snail, ii. 317, 413, 587. 
Snake-lizard, i. 448. 
Snout, i. 521. 
Snow, ied, i. 16. 
Soemmerrin^, ii. 575. 
Soils, fertili^ of, ii. .18. 
Solar light, li. 31. 
Solen, i. 222. 
Solipeda, i. 516. 
Solfy, ii, 354. 

Sores, ii. 135, 443, 505. 
Sound in fishes, i. 429. 
Sound, ii. 414. 
Spallanzani/i. 62; ii. 79, 170, 

183, 338, 567, 614. 
Spatangus, i. 205, 211. 
Spectra, ocular, ii. 525, 530. 
Spectre of the Brocken, ii. 533. 
Speed of quadrupeds, i. 496. 
Spermaceti, i. 484. 
Spherical aberration, ii. 471. 
Sphincter muscle, i. i36. 
Sphinx, ii. 217, 244, 547. 
Spicula, in sponge, i. 154. 
Spider, i. 282, 284 ; it, 248. 
Spider-crab, ii. 545. 
Spider-monkey, i. 399, 534. 
Spine, i. 387, 392. 
Spinal cord, or Spinal marrow, 

ii. 553, 604. 
Spiracles, ii. 31 1 . 
Spiral threads in plants, i. 68. 
Spiral vessels, i. 73. 
Spiral growth of plants, i. 90. 
Spiral valve in fishes, ii. 205. 
Spirits, animal, ii. 563. 
Spirula, i. 242. 
Spix, u. 252. 
Spleen, ii. 224. 
Splint bone, i. 517. 
Spokes, curved spectra of, iL 

Sponge, i. 147 ; ii. 84. 
Spongiole, i. 79 ; ii. 20, 21. 
Spotted cells of plants, i. 69. 
Spring-tail, i. 297. 
Spur of cock, i. 586. 
Squalus (see Shark). 
Squalus pristis, ii. 166. 
Squirrel, i. 524, 550; ii. 178. 
Stability of trees, i. 81. 
Stability of human frame, i. 541 . 
Stag, skeleton of, i. 507. 
Stamen, ii. 596. 
Stapes, ii. 426, 

Star-fish, i. 200 (see Asterias). 
Starcl^, i. 70; ii. 41. 
Staunton, ii. 531. 



Stearine, i. 123. 
Sieifenmtdri'u 482. 
Stems, vegetable, i. 81.. 
Stemmata, ii. 483* . 
Stentor^ ii. 98. 
Sternum, i. 402. 
Stevens^ ii. 183. 
Stigma, Fegetable, ii. 596. 
Stigmata of iniects, ii. 311. 
Sting of bee, i. 352. 
Stipuke, L 94.. 
Stomach, ii. 72, Sbc. 
Stomata, i. 77 ; ii. 19. 
Stones, Bwallowing of, ii. 171. 
Stone-wort, ii. 50. 
Stork, i. 590. 
Stratiomys, i. 310, 348. 
Straus DuTckhevny i. 300, 

323; ii.490. 
Strepsiptera, i. 350. 
Striated structures, i. 232* 
Strombus, i. 246 ; ii.301. 
Styloid bone, i. 517, 
Sttbbrachieni, i. 423. 
Suckers, i. 136, 260, 332. 
Sugar, ii. 5. 

Sun, action of, on plants, i. 9L 
Surveyor caterpillars, i. 315. 
Sus iBthiopicus, ii. 161. 
Suture, i. 381. 
Swammerdamy i. 352 ; ii. 413, 

Swan, i. 559, 593 ; ii. 169. 
dwimming of 6shes, i. 412. 
Swimming bladder, i. 429. 
Symmetry, lateral, i. 57; ii. 

Sympathy, ii. 576. 
Sympathy of ants, ii. 389. 
Sympathetic nerve, ii. 358. 
Synovia, i. 102. 
Syphon of shelly, i. 267. 
Systemic circulation, ii. 266. 

Tabanus, i. 333;. ii. 115. 
Tadpole, i. 437 r ii. 222, 322, 

Teenia, ii. 83, 114,236. 
Tail, i. 398, 524, 531 , 583 ; ii. 

392, 634. 
Talitrus, ii. 542. 
Tapetum, ii. 505. 
Tapeworm, ii. 83, 114, 236. 
Tapir, i. 521 ; ii. 392. 
Tarsus, i. 288, 328, 330, 405. 
Taste, ii. 393. 
Teeth, ii. 140. 

Tegmina of orthoptera, i. 349. 
Telegraphic eyes, ii. 493. 
TelUna, i. 224. 
Temperature, animal, ii.* 34Q. 
Tendons, i. 106, 134. 
Tendrils, i. 94. 

Tentacula, i. 161, 171 ; ii.383. 
Terebella,i. 277,278. 
Terebra, i. 249. 
Teredo, i. 235 ; ii. 295. 
TestaceUa, ii. 317. 
Testudo, i. 470 ; ii. 557 
Tetrodon, i. 420, 433. 
Textures, vegetable, i. 66. 
Textures, animal, ii. 97. 
Thetis, ii. 296. 
Thoracic duct, ii. 108, 228. 
Thorax, i. 323 ; ii. 325. 
Thorns, i. 94. 
Thought, ii. 517. 
Threads, elastic, in plants, i. 68. 
Tibia, i. 328, 330, 405. 
Tick, i. 297. 
Tiedemannt ii. 235. 
Tiger, i. 496; ii. 136, 145, 

146, 392. 
Tipula, i. 331. 
Tone, musical, ii. 419. 
Tongue of insects, ii. 124. 
Tongue, strawberry, ii. 394. 
Torpedo, i. 31 ; ii. 572. 
Tortoise, i. 463 ; ii. 499. 
Tortryx, i. 447, 448. 
Toucan, ii. 131,330. 
Touch, ii. 377, 534. 
Trachese of animals, ii. 293, 




J, L 73. 
TrarinrmtBi, iL 51. 
Tnpeziiw mofdey i. 135. 
Tremble^, L 177 ; H. 79, 478. 
Trevaramms, L 73, 75; n. 969. 
TridMclms^ i. 487. 
Tridiodm, i. 97. 
TrioD jz, L 476. 
Trirtoan, iL 113. 
TriUm^ L 252, 446. 
Trjtoiik, iL 296. 
Tritaratioo of food, intenud, u. 

Trodiaiiter, L 328. 
TrocliilM,B. 117. 
lVophi,n. 121. 
Troc, actions in, i. 494. 
Thmk-fish, i. 432. 
Tnmk of depkuH, L 520, 
Tnberote rooCi, iL 589. 
Tnbicote, L 277. 
Tnbipoffm, i. 165. 
Tnbolaria, iL 233. 
Turbinated tbellft, L 216. 
Turbinated bones, iL 400. 
Turkey, ii. 173, 440. 
Torritella, i. 249. 
TnrUe, L 463; ii. 202, 557. 
Tasks, iL 141. 
Tympaniun, ii. 422. 
Type, i. 48 ; ii. 627. 
Typhk>ps, L 447. 

Ulna, i. 405. 
Ungual bene, i. 405. 
UniobataTa,L 217. 
Unity of design, ii. 625. 
Uranoscopas, ii. 503. 
Urceolaria, L 187. 
Urchin, sea. See Echinus. 
Utricle of labyrinth, ii. 430. 
Uvea, ii. 463. 

Valves, L 31, 104 : ii. 260, 288. 
Vampire bat, ii. 117. 
Van Helmonif ii. 16. 

Vaae of feather, L 

Variety, k« < L 11, 48; iL 

Farley, iL 254. 
Vascnlar cflcaUsB, iL 235. 
Vaacaiar plena, iL 377. 
VamqmeiiMj iL 229, 336. 
Vegetable ksa^doB, L 14,4a. 
Vegetable oignaitioB, L 65. 
Vegetable mitritiDi^ iL 16. 
Veins, L 41 ; iL 108. 
Velella, L 195. 
V<4ocity of firiies, L 434. 
Vdvet coat of antler, L 510. 
Vena csv«, iL 263. 
Ventricle of heart, iL 108, 259. 
Veotridea of bnia, iL 556. 
VeietiUnm, iL 82, 478. 
Vertebcata,L361. ^ 

VertJcilkted aifangenent, i. 

Vesicles of plants, i. 66. 
Vespertilio, i 551 ; iL 136, 

Vessels of plants, L 71. 
Vesseb of animals, L 103; ii. 

606, 613, 62U 
Vestibole of ear, iL 427. 
Vibrations, ii. 563. 
Vibrio, i. 63, 186. 
Vicq d'Azyr, L 190. 
Villi, iL 347. 
Viper, i. 447; ii. 597. 
Vision, ii. 444. 
Vision, erect, u. 521. 
Visual perceptions, ii. 520. 
Vital functions, i. 38 ; ii. 1 , 69. 
Vital oi^ans, ii. 354. 
Vitality, i. 20. 
Vitreous humour, ii. 462. 
Vitreous dielb, i. 236. 
Viviparous reproduction, ii. 

Voice, ii. 444. 
Voltaic battery of torpedo, iL 




Voluntary motioo, i. 37; iu 

Volute, i. 248 ; ii. 126, 482. 
Volvox, i. 186,188; ii. 591. 
Voracity of hydra, ii. 77. 

Whelk (see Buccinum). 
Whiskers, ii. 392. 
Whorls of plants, i. 90. 
Whorls of shells, i. 243. 
Willow, i. 79'. 

VorticeUa, i. 62, 182; ii. 97, Wings, i. 344, 567. 

Vdtoxe, u. 180, 406. 

Wading biids, i. 585, 592. 

Walking, i. 492, 542. 

Waller, ii. 134. 

Walrus, i. 487 ; ii. 141. 

Warfare, animal, i. 46; ii. 67. Woodpecker, ii. 132. 

Warm-blooded circulation, ii. Woody fibres, i. 71, 75. 

Winged insects, i. 299. 
Withers, i. 518., ii. 128. 
Wollaston, i. 92; ii. 55, 491, 

Wombat, i. 527. 
Woodhouse, ii. 30. 

Water not the food of plants, 

ii. 16. 
Water-beetle (see Dytiscns). 
Water-boatman, i. 29, 337. 
Wax, TegetaUe, ii. 48. 
Web-footed birds, i. 592. 
Weber, ii. 430, 480. 
Whale, i. 55; ii. 178, 443, Zoanthns, i. 162, 182. 

Worms (see Annelida and En- 

Yarrell/ii. 131. 
Young, ii. 474, 475. 

Zebra, i. 516. 
Zemni, ii. 506. 

504, 559. 
Whalebone, ii. 136. 
Wheel animalcule, i. 189. 

Zoocarpia, i. 156. 
Zoophytes, i. 146; ii. 477, 

Wheel spokes, spectre of, ii. Zostira, ii. 202. 


Zygodactyli, i. 586. 


c wmrriftsaAB.'nioBs oocbt, chamtuit la*